The sample consisted of 393 inmates in the detoxification unit of the jail. In weekly educational sessions inmates received basic information on the human immunodeficiency virus (HIV). Specific high-risk behaviors, such as needle sharing and sexual behaviors, were discussed in detail. A pretest and posttest were administered to test inmates' knowledge about HIV transmission with an emphasis on sexual and drug-use questions. The pretest revealed that approximately 24 percent of the inmates did not know that blood is the body fluid most likely to be infected with HIV. Only 25 percent knew that receptive anal intercourse is the most risky sexual behavior. Some 50 percent of the inmates believed they could visually determine whether someone was infected with the virus. Approximately 80 percent knew that needle sharing is a dangerous activity. The posttest indicated that the educational program was effective in increasing inmates' knowledge of HIV and its transmission, but the mean test scores suggest that many inmates continued to have inadequate knowledge about HIV and its transmission. There is an urgent need to continue and expand such educational programs, attended by evaluations that improve the teaching methods used. 1 table and 15 notes