Purified, bacterially synthesized, recombinant Multi-CSF (rMulti-CSF) exhibited in vitro proliferative effects identical to those of native Multi-CSF on a broad range of progenitor cells from normal murine bone marrow and on the continuous cell line FD-CP1. The injection into normal adult mice of 6-200 ng purified rMulti-CSF three times daily for six days induced dose-related rises in peritoneal macrophages, neutrophils, and eosinophils, and in the spleen weight and content of megakaryocytes, mast cells, and progenitor cells, some responses being detectable with the lowest dose injected. No Multi-CSF was detected in the serum, organ extracts, or organ-conditioned medium from normal or endotoxin-injected mice. While the data indicate that even small doses of injected rMulti-CSF can produce detectable hemopoietic changes in normal adult mice, the pattern of these changes and the failure to detect Multi-CSF in vivo raise doubts that Multi-CSF plays a significant role as a regulator of hemopoiesis in normal adult life.