In HIV seropositive persons, secondary prophylaxis should prevent HIV-associated diseases, including opportunistic and other complications, and delay progression of HIV infection. Efforts to control transmission of secondary infections are of limited value, since these complications are mainly caused by endogenous opportunistic microorganisms. Vaccines are currently available for only a few of the pathogens that are potentially important in this clinical setting; they include influenza, Streptococcus pneumoniae and hepatitis B. However, antibody responses to these vaccines may be unsatisfactory in immunocompromised persons. Chemoprophylactic regimens have been devised primarily for prevention of Pneumocystis carinii pneumonia and tuberculosis. Recently, antiretroviral therapy with zidovudine has shown promise in delaying progression of AIDS in asymptomatic HIV seropositive individuals with impaired cellular immunity.