MIAMI (AP) _ Narcotics agents call it ''crossing the line'' or ''going south'' - succumbing to the lure of drug money. And the few who do so have created the most agonizing task the Drug Enforcement Administration faces.

Since March, three agents from the Miami office, the DEA's largest with 350 agents, have been charged with protecting the drug traffickers they were supposed to be throwing in jail.

They join three others convicted here since the early 1980s of aiding or supplying information to drug smugglers in return for hundreds of thousands of dollars.

But that doesn't mean the Medellin Cartel has somehow penetrated the DEA's top-secret operations, says John Fernandes, a DEA spokesman here.

''I don't know of any agents who were killed or any major investigations compromised by these cases,'' he said. ''These were all isolated incidents.''

Most DEA officials are reluctant to talk about cleaning their own house - the DEA's Washington headquarters failed to respond to repeated requests by The Associated Press for the number of agents arrested nationally on charges of drug involvement.

But Tom Cash, special agent in charge of the Miami office, confronts the issue head-on.

''This is elephant-land for cocaine,'' he said. ''Last year we seized $178 million in cash and property, and 40 percent of that was green.''

Cash pulled a photo from his office wall of agents posing with a $1.9 million cash haul, and pointed to individually wrapped packets of $100 bills.

''Each one of those is $10,000,'' Cash says. ''The agent thinks, 'Suppose I put two or three of these in my pocket - who's going to know the difference?'''

A spokesman for the federal Office of National Drug Control Policy in Washington says corruption in the DEA, while considered low-level, is a cause for concern.

''It's a problem we're aware of and will be looking into,'' said Larry Cirignano, spokesman for director William J. Bennett. ''The problem is money. At some point, everybody comes to a price, even though you hope they don't.''

Because of the huge amounts of cash agents deal with, and the necessity of working undercover with informants who themselves actively engage in trafficking, the DEA is the nation's most tempted law-enforcement agency, Cash said.

In response, the DEA has an unforgiving policy to combat the line-crossers, and those who allow confidential information to ''go south'' to traffickers, as the agents say.

''We don't sweep these cases under the rug,'' Cash said. ''And we do not just fire people for violations of law - we prosecute them.''

In addition, the DEA has a tight system to finger an agent who wanders from standard procedures.

Agents generally work in groups of about a dozen, and anyone who displays too much interest in a case outside his jurisdiction immediately comes under scrutiny.

The DEA keeps track of every request for its computerized files, and can later cross-check for suspicious activity if it appears traffickers know too much. There are about 2 million files in the national computer system. In Miami, the agency is currently investigating 3,000 cases and using about 500 informants.

When operating in the field, confiscated cash and drugs must be counted by at least two agents. Whenever possible, agents avoid counting cash at all and just bundle it up and send it to a bank. Because of the slippery nature of dealings with informants, no agent is supposed to meet an informant alone.

All agents - including Cash himself - must take a monthly drug test. Failure means immediate dismissal.

And every complaint against an agent, no matter what its source or how dubious, is investigated, Cash said.

''No check would ever be so thorough as when it involves a DEA agent,'' Cash says. ''We would spend our whole budget on it, if necessary.''

Cash has developed a surprising theory on which agents are most likely to cross the line.

''It's not the average special agent who is involved in these types of incidents - it's the very best. Most of them were problem-free,'' he said. ''They're the ones who are working undercover, running up against the very best on the other side.''

After a while, they believe they know the system and can beat it, he said.

The recent Miami cases bear him out. All involved agents had at least nine years of experience in the agency.

Jorge Villar, who worked with Metro-Dade police before joining DEA 11 years ago, was arrested in March for allegedly agreeing to accept $300,000 to protect a cocaine shipment from the Bahamas.

Arrested together last month were agents Drew Bunnell and Al Iglesias, who were accused of wanting $100,000 from an informant to help him hide a cocaine ripoff by disguising it as a bust. Bunnell had nine years with the DEA and Iglesias 14.