For 42 days, Dr. Manuel Limonta remembers, they closeted themselves in the laboratory, working without letup over mysterious fluids.

When they reached their goal, May 21, 1981, President Fidel Castro showed up to offer personal congratulations and suggest that someone bring out the beer for a celebration. Limonta and his half dozen colleagues drank far into the night, he recalls, toasting their accomplishment with the heavy Cuban brew.

Limonta and his laboratory team had produced Cuba's first synthetic interferon, ushering the island into a new and still tentative field of medical research. In the four years since then, Limonta's research staff has expanded to about 50 and his program at the Biological Research Center here has included extensive clinical experiments on the use of interferon against a range of diseases.

Interferon is a substance produced by the body to block the reproduction of viruses within the body's cells. Some scientists had hoped a synthetic form of the substance could cure a variety of diseases, including cancer, but much recent research has proved disappointing. Cuba's crash production of the drug drew on similar procedures in other countries in the late 1970s, making alpha interferon from white blood cells. But Limonta said his team's subsequent clinical testing has gone as far as and perhaps further than much research in other countries, where initial expectations of quick and wide use have dimmed in the light of the drug's increasingly apparent complexity.

"I think we are one of the countries with the most clinical application around the world," he said in an interview at the center. "Not many countries around the world have 2,000 cases treated by a single center . We have a national program."

Castro and other Cuban government officials have cited the interferon studies to visitors as an achievement of their rule. The Cuban leader, who has made public health an object of national pride since coming to power in 1959, has taken a personal interest in the research, periodically calling Limonta to inquire about his progress, Cuban officials and diplomats here said.

The Cuban government, eager to be recognized for its health achievements, organized an international conference on interferon research in 1983 and has begun work to hold a second next year.

Mathilda Krim, who heads the Interferon Laboratory at New York's Memorial Sloan-Kettering Cancer Center, said interferon testing by the Limonta team with a throat tumor called laryngeal papilloma has been particularly interesting to U.S. researchers. The disease is rare, she explained in a telephone conversation, and Limonta has reported using interferon on more than 80 patients.

Use of interferon has controlled or slowed tumor growth in all test patients without unacceptable side effects, Limonta said, making unnecessary the customary throat surgery to remove the growth.

"We have conquered this disease with interferon," he asserted. "In Cuba, we don't have any tracheotomies" for laryngeal papilloma.

Cuban doctors also have experimented with interferon to combat dengue hemorrhagic fever, warts, genital herpes, acute hepatitis B and several types of cancers, he said.

Results have been mixed and "we still have a long way to go," he added. "This is still something to be explored."

The center's researchers have begun producing different types of interferon, including varieties called beta and gamma, through biological engineering, Limonta reported.

Limonta, a 40-year-old physician, graduated from Havana University's medical school. He has received advanced training in Sweden, the Soviet Union and the United States.

Limonta said most of his researchers deliberately are hired when they are young, right out of the university's scientific graduate studies. By the time they are his age and moving into positions of responsibility, he said, they will have 15 years' high-level research experience.