Sex researcher Dr. William Masters thinks that he has a new way to help control the spread of AIDS. Despite the prevailing advice to engage in "safe sex," many men refuse to use condoms, placing women at risk, Masters said last week at the National Institutes of Health.

"Women should not be placed at risk of male cooperation," Masters said.

Masters' idea is to change the environment of the vagina to make it physically inhospitable to the AIDS virus. To do this, a new type of contraceptive jelly must be developed, Masters said, to increase acidity.

Scientists measure acidity with a scale called pH. The most acidic pH is 1.0, the level of acidity found in the stomach. As the pH rises, to a maximum of 14.0, it becomes less acidic and more basic.

Normally, the vagina's pH ranges from a low of 3.6 during the part of the menstrual cycle before ovulation to a high of 4.5 after ovulation.

The virus that causes AIDS thrives in a pH of 6.0 to 6.5 -- levels that the vagina reaches only in two circumstances, Masters said. The first is during the menstrual flow, when pH levels range from 5.5 to 6.0. The other is following intercourse and ejaculation, when the vagina's pH remains elevated for about six to 10 hours.

Masters' proposal: Lower the pH of the vagina to a level that will not allow the AIDS virus to thrive. "In the laboratory, the HIV virus has been reported destroyed at a pH range of 3.0 to 3.5 or below," he said.

Since sperm is also destroyed at those levels, the jelly could serve as a contraceptive as well as helping protect against the virus.

"Protection against the AIDS virus could work both ways," Masters said. If a woman were infected with the HIV virus, the vagina "could theoretically be kept relatively free" of the virus, he said, which would help protect a man. If a man were infected with the virus, a woman would also have added protected.

It is unknown how quickly such a jelly would take effect or how long its protection would last, Masters said. His appearance at NIH last Wednesday was an appeal for research funds for the idea.

Why some sexual partners of people with AIDS contract the virus after one exposure while others remain uninfected after repeated exposure is not known. But condoms can minimize the risk of absorbing an infected bodily fluid.

"Whether or not a man uses a condom, a woman is entitled to whatever protection is possible against the HIV virus," he said. "Vaginal pH control is one of these protection possibilities."