Along the flat, dry highway leading tourists toward a light-hearted Mexican holiday, a stark 12-by-24-foot billboard appeared Tuesday in the bright red, white and green of the Mexican flag.

"Warning: Not Safe to Travel to Guadalajara, Mexico."

Erected by friends of a murdered U.S. Drug Enforcement Administration agent who grew up here, the sign is only the latest and most prominent indication of trouble on what traditionally has been one of the world's more peaceful borders.

Enrique Camarena Salazar's body, wrapped in a plastic bag, was discovered March 6 about 70 miles southeast of Guadalajara, where he had been kidnaped nearly a month before.

In the meantime, U.S. Customs agents paralyzed border traffic for several days to demonstrate U.S. displeasure at the failure of Mexican authorities to track down his abductors.

His murder, as well as the disappearance of at least six U.S. tourists in Guadalajara since December and assaults and robberies of others, have led U.S. diplomats to discuss issuing official warnings against travel to some parts of Mexico, including Guadalajara.

But many leaders here are so enraged by Mexico's handling of the Camarena case that they have given up waiting for official action and are planning to take their protest along the border.

"The people in the Imperial County are sad, but they are also mad, too," said Fernando Sanga, a businessman who conceived the billboard idea.

A local sign company is moving to put a similar warning on the border crossing at Yuma, Ariz., and Sanga said he hopes to display warnings near popular border points at San Diego, Nogales, Ariz., and El Paso.

"We feel that we should do something to put pressure on the Mexican authorities in Guadalajara" to track down Camarena's killers, he said.

In Guadalajara, the attorney general's office said last night that three Mexican police commanders and 27 other people were being held in connection with the abduction and slaying of Camerena, United Press International reported.

In a tiny living room full of funeral flowers, Camarena's sister, Bertha Tamayo, described the sense of helplessness that has fueled anger here.

"Towards the fourth day after the kidnaping I began to see from the media that Mexico was not cooperating to its full capacity," she said. "If Mexico had cooperated like they should, maybe Enrique would still be alive."

His mother, Dora, a slim woman in black, sat holding the last photograph taken of her son alive.

She endorsed the billboard campaign as one way to preserve the memory of Kiki, as she called him.

"He wanted to make something good for mankind and he was paid back so badly," she said in Spanish as Tamayo translated.

Travel agents in Los Angeles report no noticeable decline in the heavy U.S. demand for Mexican vacation spots, including the museums and restaurants of Guadalajara. But Calexico City Council member Arturo Rioseco, one of several dozen sponsors of the billboard campaign, said an immediate, angry reaction from the press in Mexicali just across the border indicates that the action is being noticed.

At first glance, Calexico would seem an odd place to spearhead a campaign to pressure Mexico.

More than 95 percent of its 16,000 residents are of Mexican descent. The banks and gas stations and fast-food stops along Rte. 111 have blossomed in part because of the steady traffic to and from Mexico and the jobs available at the Customs station. Many businessmen here suffered from the strict border checks that followed Camarena's abduction.

But this is also a town of large, close-knit families and a strong Roman Catholic Church. Residents say they abhor the drug traffic and what it has done to one of their most-admired young men.

The local newspapers have been full of pictures of a lean, smiling teen-aged Camarena in his high school letter sweater. He married his high school sweetheart, Geneva, here, joined the police force here and occasionally returned from Guadalajara to visit with their three young sons. He had six sisters and two brothers with a wide circle of friends. His older brother Eduardo was killed in Vietnam in 1965, and a street here is named after him.

Enrique Camarena's memorial service was held last Saturday. That morning, in an editorial headlined "Kiki's Sacrifice," the Imperial Valley Press said, "Kiki will have died in vain" unless something is done about drug traffickers.

"A multifronted offensive must be mounted and the United States must start getting the cooperation of the supplier nations -- even if it takes such pressures as economic sanctions to get it," the newspaper said.

The city schools are planning to start a scholarship fund in Camarena's honor, but Bertha Tamayo put her emphasis on the billboard campaign and on a bill introduced by Sen. Paula Hawkins (R-Fla.) creating a $100,000 reward for information leading to the conviction of anyone who kidnaps or murders a U.S. narcotics agent.

"I know there is a lot of corruption in Mexico," Tamayo said. "I know it will take some time, some power, starting from the top to do something . . . . But I hope Mexico is more aware of what has happened, not because he was my brother, but because he was a good human being trying to do good for other people, including Mexico."