Neutrophils isolated from AIDS patients have demonstrated improved growth inhibition of Mycobacterium avium when incubated with exogenous granulocyte colony-stimulating factor (G-CSF). In this clinical study, 30 AIDS patients without M. avium infection were randomized to receive 5 days of treatment with rifabutin, G-CSF, or both agents. The M. avium killing capacity of neutrophils harvested from each patient before intervention, during (day 4), and after therapy (day 7) was assessed. The mean change in human immunodeficiency virus load in the group receiving G-CSF alone was -0.07 log of viral RNA. There was a 90% reduction in M. avium growth after therapy for patients treated with G-CSF alone (P = .01), 59% for patients treated with both agents (P = .06), and 11% for patients treated with rifabutin alone (P = .84). Thus, neutrophils isolated from AIDS patients treated with G-CSF demonstrated a significant enhancement of killing of M. avium; there was no notable effect on virus load.