A substantial number of HIV-infected persons are anergic, and thus do not respond to the only currently available diagnostic tool for tuberculosis infection (that is, the PPD (purified protein derivative) skin test). Many of these anergic persons are, however, infected with Mycobacterium tuberculosis and eventually develop reactivation tuberculosis, causing both individual illness and spread of infection to others in the community. This study examines the possibility of using INH prophylaxis (that is, for prevention) in anergic HIV-infected patients at high risk for tuberculosis as a means of decreasing the sharp rise in the incidence of tuberculosis due to HIV infection. INH is inexpensive and relatively safe, and thus may demonstrate an acceptable risk/benefit ratio as a medication that can be given over a limited period of time to a population suspected of having, but not proved to have, M. tuberculosis infection. If this study shows INH to be safe and effective in this setting, it could have a major effect on public health in this country.