The effect of recombinant human interleukin-1 (IL-1) alpha, granulocyte-macrophage colony-stimulating factor (GM-CSF), and combined factor therapy (CFT) on Rhesus monkey peripheral blood natural killer (NK) activity in vivo was compared. During a 14-day treatment period, IL-1-treated animals demonstrated a 170% increase in NK activity against K562 target cells by day 4, reaching maximal levels (300%) at day 16, and returning to baseline by day 30. NK activity of GM-CSF-treated monkeys increased slightly (60-100%) during days 4-12, as did saline-treated monkeys, but returned to baseline values by day 16. A delayed increase in NK activity resulted after GM-CSF treatment, reaching a peak (260%) on day 23 and remaining elevated through day 39. CFT resulted in a bimodal response pattern, with two peaks of NK activity: one at day 16 and a second at day 39. The first peak of activity (223%) was significantly less than the activity attained with IL-1 alone; the second peak (300%) was of greater duration and occurred later than the peak observed in GM-CSF-treated monkeys. Unlike IL-1, GM-CSF treatment did not lead to a immediate stimulation of NK activity; augmentation was delayed by more than 7 days post treatment. CFT results suggest that GM-CSF reduced the direct NK response to IL-1; while IL-1 led to an enhanced delayed NK response. Therefore, IL-1 and GM-CSF augment NK activity through different but interrelated pathways.