OS ANGELES, March 20 — Since Sept. 11, airport security officials have been armed with an array of new high-tech weapons in the battle against terrorism. Officer Michael Ahumada of the Los Angeles International Airport police force keeps the most important item in her arsenal tucked discreetly in a pocket just below her gun.

It is a tennis ball tied to a long orange string.

"Here, Jackson," she said to a dog that leaped and snapped joyfully as Officer Ahumada bounced the ball above its head in the international terminal.

"You can't do anything without this," she said. "But it's her reward for doing a good job."

With little fanfare, bomb-sniffing dogs have become among the most effective, and important, security tools at airports, federal and local airport officials say. They work quickly and have an almost perfect record in separating real explosives from merely suspicious items — which has made them one of the best friends of harried travelers desperate to avoid the security delays that mar air travel.

While billions of dollars are being spent on devices that peek inside suitcases and carry-on bags, given the new federal mandates on screening all baggage, dogs offer reassurance and are far less intimidating than some of the new measures.

In no city have the dogs made more of a difference than Los Angeles, which faces a far more serious security challenge than any other airport.

Los Angeles has the third-busiest airport in the country in terms of passenger traffic, after Chicago's O'Hare and Atlanta's Hartsfield airports, but it is the busiest in the world as measured by the amount of luggage it handles. The reason: more travelers begin their trips here, rather than merely make connections. That leaves officials with the job of screening roughly 150,000 pieces of checked baggage each day.

It is a task that frequently leaves officials with tough decisions about whether to evacuate terminals each time something unusual is found.

In recent weeks there have been several major disruptions — one when an officer forgot to plug in a security wand, one when a replica of a grenade was found in a bag, and another when a scanner mistook a coffee grinder for a bomb — each sending ripples through airports around the country because dozens of flights were delayed.

The issue is particularly significant for Los Angeles because the airport, operating well beyond the capacity it was designed for 20 years ago, generally rates poorly among travelers. J. D. Power & Associates, which runs surveys and rankings, says that travelers rate the airport here below average, in large part because it is aging and, since it is broken up into small terminals, offers far fewer shopping and eating choices than newer airports do.

Using dogs to screen luggage "has been around, but it was not really on the front burner," said Michael DiGirolamo, the airport's deputy executive director of operations. "They were valuable before, but they are much more valuable now. There's no question, the dogs are quicker and they have fewer false alarms."

Frank A. Clark, the executive director of the company that operates the big international terminal here, said: "There is no downside to these dogs. It's all positive because we've got to get away from closing terminals whenever a problem is found. The dogs are a big help in that."

The airport police here had no bomb-sniffing dogs before Sept. 11, but the Los Angeles Police Department rotated a few teams through the airport. Since then, the airport police have bought five dogs, for about $11,000 each, including training, and five more are on the way.

Jackson is a 3-year-old, female Dutch-born Belgian Malanouis, one of the most popular breeds because of its sensitive nose and cheerful manner. The airport also has some German shepherds. Federal officials said that Labrador retrievers and even standard poodles are being trained for airport duty around the country.

The Transportation Security Administration, the new federal agency overseeing airport security, estimates that the number of the dogs nationwide will rise to 300 in about two years, from 175 now, spread over 80 airports.

Officials here say the dogs may prove the salvation of travelers because they may do more to prevent unnecessary terminal evacuations and other security delays than many of the expensive new devices, like the massive CTX machines, which run a CAT scan of luggage. The airport here has 13 of the machines — each weighs about 8,000 pounds and is roughly the size of a hefty sport utility vehicle — with more on the way.

"Where they really help the most is in that gray area, when we're asking, `Should we or shouldn't we evacuate?' " Sgt. Blair Lindsay of the airport police said of the dogs. "They're so good. We always trust them."

Officer Ahumada said that on several occasions Jackson has cleared suspicious bags that, without the dog, might have prompted an evacuation. The dogs are trained to identify 13 varieties of explosive, including gunpowder and C-4.

"They're mobile and they can lead you to the source of a problem," said Rebecca Trexler, a spokeswoman for the Transportation Security Administration.

They also cheer people up. Strolling through a terminal here with Jackson was like being with Julia Roberts on a crowded city street. Nearly everyone who noticed her responded with a smile or an outstretched hand, followed by kissing sounds. Three Japanese travelers rushed over and cooed, "Kawaii," meaning, "Cute!"

"We chose these dogs in part because we want this," Sergeant Lindsay said. "People feel good around them."

Travelers said the dogs gave them a visceral sense of security that the machinery did not.

"I just thought immediately when I saw the dog, `This is very good,' " said Robert Prokop, a businessman from Germany. "Everyone likes dogs, but you know what they can do. It's safer."

Nearby, a young couple smiled at the lanky dog and gave her a little scratch.

"To me it's a lot more reassuring to see them, and they don't carry machine guns," said Kelly Farnsworth. "It's more efficient, too."

David Hicks, a visitor from New Zealand, said, "It's a great idea, much friendlier."

Even the officers said that they felt more relaxed.

"I don't know how the machines work," Sergeant Lindsay said. "Somehow I feel better knowing a brain is thinking about this and continually separating out the small things. I really have a lot of confidence in these dogs."