Longitudinal, prospective study of various aspects of cell-mediated immunity will be carried out in patient groups considered at risk for the development of severe opportunistic infections (01) as the result of a community-acquired cellular immunodeficiency syndrome known as AIDS. Emphasis will be placed on the definition of early, predictive defects in natural or adaptive cellular immunity that could be employed as diagnostic tests that permit early identification of patients. At present, no definitive test is available. The identification of such a defect would be expected to shed light on the pathogenesis of those 0I and of certain neoplastic diseases known to be complications of this immune deficiency state. Preliminary experience with AIDS indicates that one early abnormality is reflected in abnormal production in vitro of alpha-interferon (IFN). Further investigation of this abnormality will clarify its mechanism and whether it is a useful prognostic or diagnostic indicator, as well as its importance to the pathogenesis of the AIDS complications. The development of assays of other aspects of cellular immunity, particularly of T cell-T cell, T cell-monocyte interactions, and the role of IFNs in these functions, may permit further analysis of AIDS. The definition of diagnostic assay systems will further epidemiologic, therapeutic and preventive approaches to this disorder.