"Your grandparents and mine were the hardworking immigrants who used to come to American," a senior Immigration and Naturalization Service official once told a reporter, "but the sort of people who are coming now - well, it's like mixing cheap soda pop with your best Scotch whiskey."

LEONEL J. CASTILLO, who was sworn in Friday, May 13, as the new immigration commissioner, says the "cheap soda pop" image exemplifies a negative attitude toward immigrants that must be changed if the INS is to serve as an effective agency for regulating immigration to the United States.

The 37-year-old Castillo, who has been city comptroller of Houston for five years, is the first Mexican-American to hold the top INS post. Since the great waves of immigration from Eastern and Southern Europe began around 1880, the job of running the INS has never been held by anyone who shared an ethnic background with the arriving immigrants.

The largest numbers of immigrants to the United States today come from Mexico, other parts of Latin America, the West Indies and Asia. They include approximately 400,000 legal immigrants a year and an udetermined number of illegal immigrants.

"One of the biggest problems is the INS tends to look at itself only as a law enforcement agency," Castillo says. "INS doesn't exist just to keep out people who want to enter the country illegally. We also exist to provide a whole variety of services for immigrants who are here legally.

"The INS, like every police force, tends to become hardened because it sees nothing but trouble. You do get INS agents with the mentality of the immigrant as cheap soda - that goes for legal as well as illegal immigrants - and those agents become part of the problem themselves.

"Regardless of whether you're saying yes or no to an immigrant, I think it's necessary to realize they aren't all that different from our grandparents. My grandfather crossed over the Rio Grande at Brownsville in 1880, paid a nickel and became an American citizen. That's all it look. Now the economic condition of the United States is different, but the immigrants themselves have the same desires."

Castillo is taking over at INS in the midst of a growing controversy over what to do about illegal immigration. He differs significantly from the outgoing commissioner, Gen. Leondard F. Chapman Jr., in that he is not willing to venture any guesses about the number of illegal immigrants.

Castillo says he feels "like an architect who's been asked to design a building without knowing how many stories high it is. The fact is we just don't know how many illegals there are or exactly what their impact is on the economy."

He has several policy changes in mind that he hopes will make the INS more humane as well as more efficient.

One of the most important changes would be a re-deployment of INS personnel to concentrate on apprehending illegal entrants on the U.S.-Mexican border and at airports rather than in large cities. Castillo is opposed to INS activities like the roundup that took place in Washington last November at a soccer game between teams from Peru and El Salvador. The game turned into a panic-stricken melee as illegal immigrants jumped into the Tidal Basin or ran away from the INS agents.

Castillo says, "We can turn back 10 times as many people at the borders and airports - in a humane and sensible way - as we can in the interiors of urban communities. I'm not saying we're going to stop apprehending illegals in the cities, but I think we're likely to confine our activities more in response to specific complaints.

"The kind of there's been so much about in the newspapers - sweeps through blocks of apartment buildings or at soccer games of whatever - I don't see much point in it. This sort of thing causes bad feeling in minority communities, whether they're Hispanic, Chinese, whatever. It cause bad feeling among American citizens with forein accents as well as among immigrants. And it's not an efficient use of the taxpayers' money. You need a lot more INS agents to catch one illegal immigrant in an apartment in Los Angeles than you do on the border."

Some of Castillo's other for improving the efficiency of the INS in dealing with both legal and illegal immigrants include:

Standardizing alien registration cards. There are now 17 different types of cards held by more than 4.5 million people, the INS is more than a year behind in a project to come up with a uniform card.

Negotiating an agreement with the Labor Department to hire young people as clerks so that Border Patrol agents do not have to spend their time typing. The agreement would be designed to provide jobs for unemployed youths.

Bringing in "auditors" to pose as immigrants and test the efficiency and courtesy of INS offices around the country. Castillo has used the auditing technique of years in his work as comptroller in Houston.

Reading Mexican newspapers to anticipate "pressure points" along the border. "Whenever you read that there's been an earthquake or an agricultural disaster or a factory closing somewhere in Mexico," Castillo says, "you can almost predict that there's going to be a new flow of illegals at a certain point on the border. This sounds like a small thing to do, but small thing to do, but small things are often the most practical."

Castillo also wants INS officials around the country to stop testifying in favor of state bills to penalize employers of illegal immigrants. With the encouragement of outgoing commissioner Chapman, INS officials in New York and many other areas of the country have given strong public support to proposed state legislation after Congress failed to pass a bill penalizing employers.

"This is a national problem," Castillo says. "It's not just a problem on the Mexican border, it's not a problem involving immigrants from one country. We can't deal with in on a piecemeal basis, with laws framed by special-interest groups on the basis of geography or racism or some parochial economic interest."

Although Castillo disagrees with Chapman on many issues, he is grateful to his predecessor for having focused public interest on immigration issues.

"No one used to pay any attention to immigration at all," he says. "Now people do pay attention, even if the debate is sometimes pretty uninformed."