BETWEEN APRIL and October 1980, more

than 125,000 Cubans arrived in south Florida via the Mariel boat lift. Of this number, at least 22,000 had been taken from prison and put on the boats by the Castro government; others were mentally ill or aged and infirm. What has happened to these people and to the others who came of their own accord from Cuba in that mass exodus?

The surprising and impressive fact is that all but 2 percent have been resettled and are quietly earning a living and becoming Americans. This unheralded achievement is due in large measure to the efforts of the Immigration and Naturalization Service and the departments of State, Justice and Health and Human Services. Credit also is due to the many voluntary agencies that worked to find sponsors for the Cubans and that provide continuing support. Unfortunately, the remaining 2 percent of the Cubans--those who have not been resettled --are the ones we read about.

To begin with the convicts: contrary to popular belief, they have not been turned loose en masse to rape and kill on our streets, as Fidel Castro perhaps intended. Admittedly, a number slipped through the net by denying their criminal past and were processed out into the community. Hundreds have been tried in U.S. courts for offenses committed here. Some of these offenses are serious felonies, such as those described in a recent article in this newspaper.

But these numbers must be considered in context. Of those Cubans who came in the boat lift, 22,000 admitted to immigration authorities that they were convicts. They were segregated and eventually gathered in a special facility at the Atlanta penitentiary. Each was then interviewed at least three times. Those who had been convicted of violent crimes, recent crimes involving moral turpitude or multiple offenses of any kind were kept in Atlanta. The others--some of whom had been political prisoners in Cuba--have gradually been released to the supervision of American sponsors.

Only 1,500 of the most serious offenders remain in Atlanta awaiting deportation. Since Fidel Castro will not take them back, they may have a long wait. But their cases are regularly reviewed, and American lawyers are active in their behalf to ensure that they receive all the rights to which undocumented aliens are entitled. We do not have to accept them as immigrants and, in the absence of a court order, do not have to release them. To do so at this time, and after this extensive screening process, would be reckless.

The other problem group--the mentally ill and those with severe personality disorders--is now at Fort Chaffee, Ark., and will soon be moved to a federal facility in Montana. Of these 400 or so, it is estimated that about half will eventually be sponsored and released. Others will be cared for in institutions.

Fidel Castro undoubtedly thought he would embarrass and discredit our government by unloading his undesirables on the beach at Key West. He must be disappointed. Americans have accorded the migrants both charity and justice.