Eight AIDS patients with Mycobacterium avium complex (MAC) bacteremia were randomized to receive azithromycin with or without granulocyte-macrophage colony-stimulating factor (GM-CSF) for 6 weeks to examine the effect of GM-CSF administration on clearance of mycobacteremia and on monocyte function. Superoxide anion production was significantly increased ex vivo in monocytes from patients receiving GM-CSF but not in those from patients receiving azithromycin alone. Relative to monocytes obtained from untreated healthy controls, median differences in viable intracellular MAC at 2, 4, and 6 weeks were -0.76, -0.94, and -0.47 log10 cfu/mL of lysate for cells from patients receiving GM-CSF versus -0.15, -0.11, and -0.19 log10 cfu/mL for cells from patients receiving azithromycin alone. Although no effect on mycobacteremia was detected, the administration of GM-CSF to AIDS patients with MAC bacteremia resulted in activation of their blood monocytes, as evidenced by increased superoxide anion production and enhanced mycobactericidal activity. GM-CSF deserves further investigation in the treatment of mycobacterial infections.