Using a nonhuman-primate model of radiation-induced bone marrow aplasia, we examined whether the single, concomitant, or sequential administration of recombinant human interleukin-3 (IL-3) and IL-6 would promote bone marrow regeneration measured by an increase in circulating platelets (PLT) and neutrophils (PMN). Rhesus monkeys were irradiated at 450 cGy and were randomly assigned to one of five treatment protocols, receiving IL-6; IL-3; combined IL-6 and IL-3; sequential IL- 3 and IL-6; or human serum albumin (HSA) as a control. Cytokines or HSA were administered at total dosages of 15 micrograms/kg/day. Complete blood counts and white blood cell differentials were monitored for 60 days postirradiation. Both IL-3 and IL-6 significantly enhanced the regeneration of PLTs and decreased the duration of thrombocytopenia (P = .005) without affecting PMN recovery. The radiation-induced anemia that was observed in the HSA-treated controls was less severe and resolved more quickly in the IL-6 treated animals. Sequential IL-3/IL-6 significantly increased the production of PLTs when compared with the HSA-treated controls (P = .003) and monkeys receiving concomitant IL- 3/IL-6 (P = .041) but did not alter PMN levels significantly (P = .80). Coadministration of IL-6 and IL-3 did not enhance PLT but improved PMN recovery over IL-6 alone. In this primate model of marrow aplasia, IL-6 significantly enhanced the regeneration of PLTs but had no significant effect on PMN production, and did not exacerbate radiation-induced anemia. Furthermore, the use of sequentially administered IL-3 and IL-6 may improve PLT recovery as compared with concurrent IL-3/IL-6 administration, although this protocol is not significantly different in effect than either cytokine alone.