rhIL-15 administration reduces the circulating absolute neutrophil numbers in rhesus macaques. Vehicle diluent or rhIL-15 at 10, 20, and 50 μg/kg/d was administered daily for 12 days to rhesus macaques. The circulating neutrophil numbers were determined before therapy and on days 8 and 13 for animals killed on day 13, and before therapy and on days 8, 13, 15, 22, 29, and 48 for those killed on day 48. Three of 6 animals receiving 20 μg/kg/d and 3 of 6 animals receiving 50 μg/kg/d of rhIL-15 had a grade 3/4 reduction in circulating neutrophil numbers. These reductions were transient, with a return toward normal levels by 72 hours after cessation of rhIL-15 administration.