Italian researchers have found a fifth virus in the family of viruses that causes leukemias and AIDS in humans.

The new virus, called HTLV-5, has been linked to a rare kind of leukemia that causes skin eruptions, called mycoses fungoides. The virus also appears to react partially to the AIDS-antibody tests, according to a report in Science magazine published today.

About 1,000 cases of mycoses fungoides are reported in the United States annually. It is not known what percentage of these might be associated with HTLV-5, if the link is confirmed.

Viruses with the designation HTLV, for Human T-lymphotropic Virus, attack the white blood cells called T lymphocytes. These cells are the key cells responsible for the body's defenses against microbes. When infected by the HTLV family of viruses, the T cells can become cancerous and spread through the blood and lymph systems. They also can fail to defend the body.

HTLV-3 is the virus that causes AIDS. It falls in a branch of the HTLV family of viruses that is immune-system damaging. Another branch is chiefly cancer-causing.

"HTLV-5 bridges the gap between the two kinds of viruses, the leukemia viruses and the immune-deficiency viruses" of the HTLV family, said Dr. Zaki Salahuddin of the National Cancer Institute. It appears to have some of the properties of each variety of virus.

The find was reported by a team led by Vittorio Manzari at the University of Rome, who wrote that the virus was spotted in seven patients with mycoses fungoides.

In addition, the wife of one of the patients also was found to be infected with the virus, though she showed no symptoms. Her infection suggested that the virus may be transmitted sexually, a common property for the AIDS virus but not thought to be common among the cancer-causing viruses.

The HTLV family of viruses is the first group of viruses shown to cause cancer in humans. It is also in the category of viruses called retroviruses, which reverse the usual method by which viruses multiply in the cell. When retroviruses were discovered they were found to be the cause of a number of previously known, but little understood, diseases in animals and humans.

All the HTLV discoveries have come since 1980. The first two were in Dr. Robert Gallo's laboratory of tumor biology at the National Cancer Institute. The third, the AIDS virus, was found in Gallo's lab as well as the lab of Dr. Luc Montagnier of the Pasteur Institute in Paris.

The HTLV cancer viruses -- called HTLV 1, 2 and now 5 -- are geographically concentrated and cause very little disease in one area of the world while causing much more in some other places.

For example, HTLV-1 infects more than 10 percent of the population in southwestern Japan. About 200 people per year are found to have symptoms of the disease there, while fewer than 100 have ever been diagnosed in the United States.

HTLV-1 is apparently transmitted through breastfeeding but long remains dormant. It causes a fierce and deadly leukemia in one of 10,000 of those infected. The onset occurs on average at age 45.

Another hot spot for HLTV-1 is the Caribbean, while HTLV-5 appears more common in part of Italy than other places.

Viruses similar to the HTLV family, causing similar diseases and found to have similar genetic properties, have been found in cats and monkeys. And there are similarities to other families of virus that cause disease in sheep, goats and horses.

It was the discovery in Gallo's lab of the HTLV-1 virus and symptoms associated with it including Pneumocystis pneumonia that led in 1982 to the search for a retrovirus as the cause of AIDS.