Monocytes and macrophages play an important role in the pathogenesis of the acquired immunodeficiency syndrome (AIDS). Dysfunction of these cells contributes to the immunocompromised state that characterizes AIDS, and they probably serve as a reservoir for the human immunodeficiency virus (HIV). HIV-infected macrophages may also be responsible for the infection of many CD4-positive lymphocytes by means of cell to cell contact during the initiation of immunological responses. The efficiency of this process would be enhanced by activation of the macrophages, since that is accompanied by increased expression of class II major histocompatibility (HLA-DR) antigen. Using a direct blood antibody marking procedure in conjunction with flow cytometry, we have analyzed the expression of HLA-DR antigen on the surfaces of monocytes present in the peripheral blood of HIV-infected patients. In contrast to the results reported by other investigators who purified the monocytes using Ficoll-Hypaque centrifugation prior to antibody marking, we found that the percentage of monocytes expressing HLA-DR antigen was identical i n the patients and normal controls. However, a subpopulation of monocytes was detected in the blood of the majority of the patients that was expressing increased levels of HLA-DR antigen. It was also found that the proportion of monocytes with a high density of HLA-DR antigen on their surfaces negatively correlated with the absolute numbers of CD4positive lymphocytes present in the peripheral blood of the patient. These findings support the postulated role of monocytes and macrophages in the HIV infection and ultimate destruction of CD4-positive lymphocytes.