Monocytes and neutrophils are involved in the primary immune response against opportunistic infections that occur during the progression of human immunodeficiency virus (HIV) infection towards development of acquired immune deficiency syndrome (AIDS). Phagocytic cells operate through the generation of reactive oxygen species which may be toxic for fungi, bacteria and viruses. In the present study we evaluated the function of monocytes and granulocytes in whole blood samples of 16 healthy controls, 12 HIV infected subjects who had not undergone significant infections and of 17 individuals with AIDS. Using flow cytometric methods we were able to determine phagocytosis and respiratory burst under conditions that reflect the normal environment of these cells. Compared with results in samples from controls, granulocytes and monocytes from asymptomatic HIV infected patients exhibited a significantly increased capacity to phagocytose bacteria. The production of reactive oxygen intermediates was in the normal range. In comparison to asymptomatic HIV infected individuals, patients with AIDS showed a significant reduction of phagocytosis and respiratory burst which correlated with the number of CD4+ cells. In comparison to controls, patients infected with HIV, whether they were symptomatic or not, revealed a significantly diminished number of oxygen radical producing cells compared with the number of phagocytic cells. These results indicate that monocytes and granulocytes show reduced antimicrobial activity even in early stages of HIV infection. This defect is only partly due to the HIV infection itself as neutrophils are not target cells for HIV.