Alcohol and other drugs could be physiological risk factors for the development of AIDS in two different ways: they could increase the risk of primary infection when the individual is first exposed to the HIV; and for individuals already infected with HIV, they could depress the immune mechanisms which act to limit its negative impact, resulting in progression from asymptomatic to clinical infection. Considerable clinical evidence suggests that alcoholics are more susceptible to infection than are nondrinkers. In addition, numerous clinical clues suggest that alcohol consumption may interfere with normal host responses to infection. Available data regarding the effects of drugs such as opiates, marihuana, stimulants, barbiturates, and amyl nitrite on the immune system are reviewed. 4 tables, 113 notes.