Bill Slattery got involved in the nation's immigration problems almost by chance.

It was 1970, and he was being released in Yuma, Ariz., after a five-year stint in the Marines. He heard that some of his buddies were signing up for jobs with the Border Patrol, a branch of the Immigration and Naturalization Service, and decided to go along. "I had seen some Border Patrol operations out there. I guess it whet my appetite," he said.

Today, Slattery finds himself in perhaps the toughest job at INS.

As assistant commissioner for legalization, he is at the heart of a program expected to attract up to 4 million illegal aliens seeking amnesty and permanent residence in the United States under the immigration law signed last November by President Reagan.

When the one-year legalization program started in May, it was up to Slattery to see to it that more than 100 legalization offices were opened across the country, complete with workers and equipment.

When immigration lawyers complained that forms had not been delivered to the offices by the start of the program, Slattery took the heat.

"We had to create regulations. We opened up 107 offices nationwide with more than 2,000 employes. We had to figure out where to put them, had to create application forms, create identification cards. I'm not sure McDonald's could have done it," Slattery said. "The newspapers had already promised it would be chaos and confusion. But we did it. We opened on May 5."

As the program moves along, Slattery will be the one to make uncomfortable decisions about what to do when some members of a family qualify for amnesty and others do not. Only those who have lived here continuously since before Jan. 1, 1982, meet the law's requirement for amnesty.

Children born in the United States are automatically U.S. citizens. But in many cases, one parent may qualify, while the other does not.

Slattery said that in most instances he thinks family members who do not qualify are remaining underground, rather than returning to their homeland.

"I don't think we've broken up a single family. I think that whole issue is overblown," he said. "The application is confidential, and any information that would identify an ineligible family member is not shared with INS enforcement. Each family will have to make its own call, but they have a better chance of getting the whole family in eventually if some of them become legal residents.

"There's no evidence that ineligible family members are being scooped up and expelled. There's a lot of smoke and noise, but there's no way these people are going to be any worse off under the new law than they were before."

Slattery, who now tangles with members of Congress, lobbyists and reporters, spent his early federal days along the Texas border, hunting for illegal aliens and shipments of marijuana.

Born in Newark in 1947, Slattery joined the Marines right out of high school. He later was accepted for the Border Patrol and assigned to Hebbronville, Tex., a hot and dusty outpost in the heart of Jim Hogg County, about 50 miles east of Laredo.

He spent five years there, chasing illegal aliens and drug smugglers, learned to speak Spanish and received a special commendation in 1973 when the Hebbronville office was found to have intercepted more contraband per agent than any other Border Patrol office.

Slattery was transferred to Philadelphia as a special agent in 1974 and moved on to Newark in 1977, where he served as a special agent supervisor. Later, he was promoted to assistant district director for deportation. About that time, he enrolled for night courses at Middlesex County College.

In 1980, at the height of the crisis over Cuban immigrants landing on Florida's beaches, Slattery was sent on an ill-fated mission to Puerto Rico to help open a large government detention center for the Marielitos.

"The government of Puerto Rico wasn't fond of the idea of taking all those Cubans," Slattery said. "There were lawsuits, restraining orders. Then {President Jimmy} Carter lost the election, and the whole project died."

In early 1981, he was assigned to New York City as deputy assistant district director for investigations. At Kennedy airport, he supervised a unit that broke up a ring that smuggled babies from Brazil through New York to Israel for purchase.

He moved up again, to assistant district director for examinations, and led an investigation into marriage fraud. CBS News' "60 Minutes" featured the case of Dulce Llarvias, who acknowledged that for a fee she arranged hundreds of sham marriages for illegal aliens from the Dominican Republic who were seeking to become U.S. residents.

Slattery was transferred to Washington last November, just after Reagan signed the immigration package into law. Since then, he has tried to stay one step ahead of the criticism over the amnesty program's start-up.

"I'm not angry about all that," he said. "There's a certain personal satisfaction that we did as well as we did despite all their predictions. But the games aren't over yet. I guess we've still got plenty of time to screw up."

The workload has cut into his recreation time. The boat he uses for water-skiing is parked in a garage in New Jersey. But Slattery is taking time for the beach this summer, and he has planned three ski trips for this winter with his two sons.

Slattery plans to stay with INS for the long haul. The agency, he said, has given him opportunities he would not have found elsewhere, especially starting without a college degree.

"The service has been very good to me. You'd have to pry me out of here."