Blood samples from a series of 12 patients with Kaposi's sarcoma or infectious complications of the acquired immunodeficiency syndrome (AIDS) and from 18 homosexual contacts of AIDS patients were screened for interferon-related tubuloreticular inclusions (TRI) in circulating leukocytes. In the AIDS patients, TRI were detected by transmission electron microscopy in 1.5 to 10% of mononuclear cell sections. They were most frequent in patients with a decreased fraction of T helper cells and T4/T8 ratios less than 0.2. Only rare TRI-positive sections were found in 3/12 homosexual contacts with lymphadenopathy and 1/6 asymptomatic contacts. Serum interferon was found to be elevated in each AIDS case tested, but was not a sufficient condition for detection of TRI in homosexual contacts. Active DNA virus infections, including cytomegalovirus, were common to the AIDS cases and possibly contributed to the TRI pathogenesis. Localization of TRI in T suppressor/cytotoxic cells was demonstrated with monoclonal anti-Leu 2a antibodies. The pathophysiologic significance of interferon stimulation with formation of TRI in immunocompetent cells requires further investigation.