FEDERALSBURG, MD., AUG. 30 -- All summer long, social worker Beatrice Rivera has been pleading, cajoling and warning migrant workers and their employers to meet Tuesday's deadline to get a physical examination required for legalization under the new immigration law.

She persuaded a Washington resident, Dr. Jean T. Pouyes, to go to the Eastern Shore on Sundays to conduct the physicals after none of the dozen or so physicians who practice in the two counties that make up Virginia's Eastern Shore was willing, or able, to do them. Rivera even uses her own car, on a day off, to accompany the migrants on the 100-mile trip from her home base in Northampton County, Va.

"If I want it done, I've got to do it, whether it's required or I'm paid or not," she said. At first, "many were frightened to come in. They thought I would line them up and send them back."

The problems of qualifying seasonal agricultural workers -- migrants -- for possible citizenship are staggering.

In addition to overcoming language barriers and the immigrants' fears, employes at the Delmarva Rural Ministries' clinic here must untangle often conflicting instructions from the Immigration and Naturalization Service.

For example, according to Vern Jarvis, the Washington-based spokesman for the INS, the Tueaday deadline is virtually meaningless.

"The real deadline isn't until Dec. 1, 1988," Jervis said in a telephone interview last week. That's the first time the government can impose sanctions against employers who hire illegal aliens, he explained, and before then, INS is not going to be conducting raids.

Rivera, who is, in the bureaucratic jargon of the INS, a QDE -- or a "qualified designated entity," she explained with a laugh -- said she was relying on instructions from INS's regional office in Arlington.

Sure enough, over in Arlington, William R. Tillman, the INS acting deputy director for Virginia, said that "Tuesday's deadline is a crisis" for migrants, because that is when "the special rules provision expires;" it had given them time to produce evidence that they are eligible to apply for citizenship.

Rivera is not surprised about the conflicting information.

To become a QDE, she was sent to seminars in New York, Philadelphia and Dover, Del., but she said, "I quit going because each time they had a new set of rules."

The law appears simple enough, as spelled out on a bilingual -- Spanish and English -- calendar posted at Delmarva's clinic in Nasawaddox, Va., where Rivera works: Aliens who lived in the United States illegally before Jan. 1, 1982, and those who lived here between May 1, 1985, and May 1, 1986, and worked in agriculture for 90 days during that 12-month period may qualify for legal status.

This summer, Rivera said, she asked the growers to help their workers fill out the applications, but the farmers had no training and were not anxious to take time out from the busy harvest season.

Some crew leaders -- the savvy entrepreneurs who transport the migrant workers from Florida -- were willing to help, but they made mistakes that resulted in INS returning the forms.

It is a complicated, and expensive, procedure for the migrants.

To begin with, they must secure verification of employment, in the form of an employer's signature or pay stubs, the latter of which, Rivera said, many migrants discard as they move from place to place, sometimes not even knowing where they had been.

"I understand," Rivera said. "I was in the migrant stream one time myself," for six years in the 1960s, after, at age 15, she married a Puerto Rican who worked the fields in her native Virginia.

"Someone who has never been in the stream may find it hard to believe, but if a bus pulls up and the driver offers work, I don't need his name, he doesn't need mine, and neither cares."

Migrants also must be fingerprinted, which the sheriff's departments in Accomack and Northampton counties did without charge, and photographed ($16 for two prints at the camera shop in Exmore, Va.). The hospital in Nasawaddox took the required X-ray ($32 plus $14 more to have it read) and blood test, about $16.

After that, the migrant stream has led to the one-story brick building here on Main Street, where, for $50, Pouyes examines them.

Pouyes, who was born in Haiti, acknowledged that some people have criticized him for making a pile of money -- he has examined as many as 100 migrants in a day, which totals about $5,000 -- from those less fortunate than himself, including many countrymen. But he said, "not all doctors are rich. I've got expenses, and some Sundays, only a handful show up."

Pouyes said the general physical, including tests for various venereal diseases (but not for AIDS until Dec. 1), mental retardation, sexual deviation, psychopathic personality and narcotic and alcohol addiction, takes about 15 minutes. (A public health physician estimated it would take her about 45 minutes, and said an internist said it would require 90 minutes.)

When Pouyes detects a condition that requires treatment, he said, he refers the patient to the local public health department. "But their general health is good," he said. "If it were not, they would not be able to do the kind of work they're doing."