Recombinant human interleukin 6 (rhIL-6) was administered s.c. every 12 h at a daily dose of 10 micrograms/kg body weight to normal healthy mice. After 4 days the numbers of progenitor cells (erythroid burst-forming units, BFU-E and granulocyte-macrophage colony-forming cells, GM-CFC) were significantly increased (p less than 0.01) in the bone marrows and spleens of treated animals. There was no significant change in spleen colony forming unit (CFU-S) number, whereas mixed-lineage colony-forming cell (Mix-CFC) number was elevated only in bone marrow. The number of nucleated cells in peripheral blood was increased in rhIL-6-treated mice, resulting from a significant (p less than 0.01) increase in neutrophil numbers and a decrease in lymphocyte numbers. The number of platelets in these animals was also higher than in controls (p less than 0.05). These results suggest that rhIL-6 is an effective stimulator of unipotent hemopoietic cells of myeloid, erythroid, and thrombocytic lineages when administered in vivo to mice and indicate a possible therapeutic potential of IL-6 in clinical situations.