Interleukin-1 (IL-1) produces many direct and indirect effects in hematopoiesis. IL-1 stimulates hematopoietic progenitors, and enhances production of other cytokines. Experimental data suggest a role in ameliorating the myelosuppression that occurs following cytotoxic chemotherapy. Recombinant human interleukin-1 has now entered clinical trial. We conducted a phase I trial of IL-1βalone, and following a myelosuppressive dose of 5-fluorouracil in patients with cancer. A prominent neutrophilia was noted within several hours, with a later, more sustained thrombocytosis. Hypotension was dose-limiting. We observed fewer days of neutropenia following 5-FU plus IL-1 compared to IL-1 alone, but the difference did not reach statistical significance. Early results from other investigators suggest a possible protective effect from chemotherapy-induced myelosuppression. Preclinical data suggest that combinations of IL-1 and lineage specific growth factors may produce synergistic hematologic effects.