We investigated the effects of recombinant human granulocyte-macrophage colony-stimulating factor (rGM-CSF) administered by the subcutaneous route, first alone and then alternating with azidothymidine (AZT), in leukopenic patients with severe human immunodeficiency virus (HIV) infection. Ten patients with acquired immunodeficiency syndrome (AIDS) or related disorders, five of whom could not tolerate conventional doses of AZT, were administered rGM-CSF subcutaneously for 12 days. They then were administered an alternating regimen using AZT for 1 week, followed by 5 days of subcutaneous rGM-CSF and 2 days without any medication. During the initial 12 days of GM-CSF administration, there was an increase in the mean white blood cell (WBC) value. In addition, rGM-CSF stimulated circulating monocytes as evidenced by an increase in superoxide anion production and expression of surface HLA-DR antigen. However, at the same time rGM-CSF increased the serum HIV p24 antigen in each of the six evaluable patients from 189 x/divided by 2.02 pg/mL (geometric mean x/divided by SEM) at entry to 375 x/divided by 2.11 pg/mL (P less than .05). During the subsequent period of alternating AZT and rGM-CSF treatment, serum HIV p24 antigen fell below the day 14 value in most patients, particularly after the weeks of AZT administration. The mean T4 cell value increased in patients who had not previously received AZT, but generally did not change in those who had prior AZT exposure. Hematologic toxicity appeared to be somewhat reduced compared with continuous full-dose AZT therapy, and two patients with previous AZT hematologic toxicity tolerated this alternating regimen for 25 weeks. Additional regimens simultaneously combining these two agents are worth exploring.