This study assesses the changes in the expression of histocompatibility antigen (HLA)-DR by mononuclear phagocytes from HIV-infected individuals. Overnight culture of monocytes resulted in an increase in HLA-DR expression by monocytes from uninfected individuals. In contrast, the expression of HLA-DR by monocytes from HIV-infected patients decreased spontaneously and was most pronounced in patients with clinical AIDS. We also found thatMycobacterium aviumgrew within monocytes from patients infected with HIV. The correlation between major histocompatibility complex (MHC) class II expression and Mycobacterial growth which has been reported in mice was not observed in monocytes from HIV-infected patients.