Eligibility for the study will depend on age, NCI officals said. Any woman over 60, the age at which the risk of developing breast cancer becomes greatest, will be accepted; so will women between the ages of 35 and 59 who have two or more of the factors known to increase the risk of breast cancer in younger women, including a close relative who suffered from the disease or previous biopsies showing benign cell growths.

Leslie Ford, chief of NCI's Community Oncology and Rehabilitation Branch, said that the rat data were somewhat misleading. The rats were given does of the drugs far greater than those planned for women in the trial, she said. Rat livers also have many more cells capable of processing estrogen that do human livers. Thus it is not surprising that the drug's estrogenic tendencies should have a much more profound effect on that animal's liver. By contrast, she noted, mice have far fewer estrogen receptors in their livers, and a recent 15-month mouse study, in which the animals were given tamoxifen doses the equivalent of 100 times the standard human dose level, produced no liver cancers. Human livers have even fewer estrogen receptors than do those of mice.

In view of the potential risks, however, NCI will limit participation to women who are at clear risk for developing breast cancer. According to a statistical model developed by NCI, the key risk factors for breast cancer appear to be: having at least one close relative who had breast cancer; beginning menstruation before age 12; having few or no children or having children past the age of 30; and having a history of benign but "disorderly" cell growth in the breast.