Oxidative burst responses of monocytes and monocyte-derived macrophages (MDM) were studied in 40 subjects with HIV infection of different clinical stages. Oxidative burst was assessed as reduction of nitroblue tetrazolium (NBT) with or without stimulants. Results were determined as oxidative burst responses per cell and as a stimulatory ratio between stimulated and unstimulated NBT reduction. Cells from 12 HIV-seronegative homosexual men and 38 blood donors served as control groups. In patients with asymptomatic HIV infection, monocyte oxidative burst responses were reduced compared with the blood donors. In MDM from the same patients, stimulatory ratios were reduced. In AIDS patients, stimulatory ratios of both monocytes and MDM were reduced compared with controls. In contrast to the progressive deterioration of CD4+ lymphocyte counts as well as other immune functions in HIV infection, monocyte oxidative burst responses are impaired already in the asymptomatic phase of the infection, almost to the same extent as in patients with AIDS.