American and French researchers, in separate announcements, said yesterday that they have isolated new types of viruses from the blood of West Africans that are related to the virus known to cause acquired immune deficiency syndrome (AIDS) in humans worldwide.

Drs. Max Essex and Phyllis Kanki of the Harvard School of Public Health said they had found signs of a new virus in blood from more than 40 apparently healthy individuals in Senegal and isolated it from three female prostitutes in the capital Dakar.

Kanki said "definitive evidence" exists that the new, apparently harmless human virus is distinct from the lethal AIDS virus, although both appear to share common parts and infect human white-blood cells crucial to the immune system. She said this may spur efforts to develop a protective vaccine against AIDS.

The Harvard researchers said the new human virus may be a "missing link" between one they found earlier in healthy African green monkeys and the lethal human AIDS virus.

It may provide important clues to the origin of this family of viruses, they said, and reveal what characteristic of the AIDS virus causes the lethal disease in humans.

The Harvard team is to present its findings at a meeting of the American Society for Microbiology here today, and it is scheduled for publication in Science magazine early next month.

Public announcement of the Harvard findings was hastened by news reports yesterday about work at the Pasteur Institute in Paris.

French researchers said they had discovered a distinct new form of the AIDS virus that may be responsible for rare forms of the disease in two West African AIDS patients, male heterosexuals seeking treatment in Europe.

The Harvard and Pasteur Institute reports are expected to attract widespread attention in the scientific community, but spokesmen at both laboratories said they cannot yet link the two new types of viruses found in the West Africans.

Dr. Luc Montagnier of the Pasteur Institute presented his findings to reporters and a scientific meeting in Lisbon, and a New York public relations firm issued a brief news release about them.

Dr. Caroline Chaine, a Pasteur spokeswoman reached last night by telephone, said the research has been been submitted to Science magazine but not yet accepted for publication.

She said the virus found by the French team was not found in healthy West Africans but, like the Harvard discovery, appears to be more closely related to the virus found in African green monkeys than to the known AIDS virus.

The AIDS virus is known internationally by two names, based on separate discoveries by rival French and American laboratories. Montagnier first named it LAV, or Lymphadenopathy Associated Virus, and dubbed his newly discovered form LAV-2.

The AIDS virus was also discovered by National Cancer Institute researcher Dr. Robert Gallo, who named it HTLV-3, or Human T-Lymphotropic Virus-3, and developed a widely used blood test to find signs of infection with the virus. The Harvard group called its new virus HTLV-4.

"The announcement by Montagnier is unorthodox at best. We don't know if it's the same virus Dr. Essex has been talking about openly for four months at scientific meetings," Gallo said yesterday.

Kanki said that, in the screening of prostitutes and others in Senegal, the Harvard team found no evidence of illness associated with the HTLV-4 virus and that, to her knowledge, AIDS has not been detected in that nation. Widespread AIDS has been reported in central African countries such as Zaire.

The Harvard group has speculated that monkeys may have transmitted AIDS-linked viruses to humans in Africa. Kanki said isolation of the new human form will allow "comparative studies" of all the viruses.