The effects of recombinant human interleukin-1 rhIL-1 beta (rhIL-1b) on hematopoietic recovery following chemotherapy in a primate model were investigated. Cynomolgus monkeys received 1 microgram/kg/day rhIL-1b intravenously for 2, 7, and 14 days following 5-Fluorouracil (5-FU) treatment (75 mg/kg x 2 days). Compared with controls, a significantly shortened time to achieve an absolute neutrophil (ANC) count over 500/microL was observed in animals receiving 2- and 7-day courses of rhIL-1b (17 v 30 days), while animals receiving a 14-day course of rhIL- 1b achieved an ANC over 500/microL by 23 days. Concomitantly, a marked increase in granulocyte-macrophage colonies (CFU-GM) was observed at 14 days following 5-FU in animals receiving 2- and 7-day rhIL-1b courses. In animals receiving a 14-day rhIL-1b course, a significant increase in CFU-GM relative to control was not seen until 21 days post 5-FU. Utilizing a serum-free colony assay system, a 50% inhibition of normal marrow CFU-GM growth was observed with the addition of sera obtained on day 9 post 5-FU from animals receiving rhIL-1b for 14 days. Sera obtained at any time from animals receiving 2- and 7-day rhIL-1b treatment did not show any growth inhibition. Addition of antibodies to TNFa to the coculture assay abrogated the CFU-GM growth inhibition. TNFa levels in sera with the inhibitory activity was relatively high (918 pg/mL). Our data indicate that rhIL-1b enhances hematopoietic recovery following 5-FU if administered for short periods of time (less than 7 days), whereas prolonged administration has a counterproductive effect that is due in part to the induction of TNFa production.