Recombinant human interleukin-3 (rhIL-3) was administered to 30 patients undergoing autologous bone marrow transplant (ABMT) for treatment of lymphoma. In this phase I dose escalation study, rhIL-3 was administered from day 0 to 20 after ABMT by 2-hour intravenous infusion at dose levels of 1, 2, 5, and 10 micrograms/kg/d. Seventeen patients did not complete therapy with rhIL-3. Eleven requested early discontinuation for malaise, confusion, transplant complications, or rapid engraftment and were removed from the study, whereas six patients developed grade III toxicity, including fever (three patients), or headache (three patients) possibly attributable to rhIL3. Other common toxicities included diarrhea, rigors, mucositis, and rash. The maximum tolerated dose of rhIL-3 was 2 micrograms/kg/d. No evidence of earlier hematopoietic cell recovery was observed compared with similar historical patients treated with recombinant human granulocyte- macrophage colony-stimulating factor. Future trials will be needed to determine alternate schedules of administration of rhIL-3 or the use of rhIL-3 in combination or in sequence with other growth factors.