We examined human immunodeficiency virus (HIV) production in cultured mononuclear cells from 36 seropositive homosexual males. Production was detected by using an HIV p24 antigen ELISA assay in blood mononuclear cells in 54% of asymptomatic, 71% of acquired immunodeficiency syndrome (AIDS)-related complex, and 100% of AIDS patients. When the peripheral blood mononuclear cells were separated into monocytes and CD4+ T cells, we found that the CD4+ T-cell fraction was preferentially infected in the three clinical stages. The ability to isolate HIV from blood monocyte-derived macrophages was similar in the three stages (24-33%) and required coculture with phytohemagglutinin-stimulated lymphoblasts. Bone marrow and blood mononuclear cells cultured simultaneously yielded virus from both sources in 13 individuals. Again the prime source of virus was the nonadherent bone marrow mononuclear cells, which contained CD4+ T cells, and not the adherent monocyte-enriched fraction. We conclude that blood mononuclear cells yield productive virus more readily as disease progresses and that infection is detected in association with CD4+ T-cell-enriched fractions. In our large sample of patients, monocyte infection was detected in only a small fraction, suggesting that this cell type is neither a primary nor an exclusive reservoir of HIV infection.