The in vivo effects of recombinant human interleukin 6 (IL-6) on the hematopoietic system of sublethally (4.8 Gy) x-irradiated mice were investigated. Animals were injected twice daily s.c. with IL-6 (10 micrograms/kg body weight/day) for 7 days following irradiation, and the numbers of hematopoietic stem, progenitor, and circulating blood cells were evaluated at 4, 7, 13, and 23 days. IL-6 caused significant depression of early hematopoiesis (decreased numbers of spleen colony-forming units [CFU-S] and granulocyte-macrophage colony-forming cells [GM-CFC]) in the spleens of irradiated mice. Marrow hematopoiesis was less affected by IL-6 injection, although the number of hematopoietic cells was also significantly lower than in irradiated mice injected with carrier alone. The observed decrease in the numbers of hematopoietic cells was not reflected by any significant change in the circulating blood cell numbers, which were similar to those in control irradiated animals. In contrast, IL-6 administered in 100 times higher doses (1000 micrograms/kg/day) caused significant increases in bone marrow and spleen cellularity and GM-CFC numbers, thus accelerating postirradiation hematopoietic regeneration. Our studies show that IL-6, administered in relatively low doses, suppresses postirradiation hematopoietic recovery, decreasing the numbers of stem and progenitor cells in sublethally irradiated mice.