Normal adult BALB/c, C57BL, and C3H/HeJ mice were injected intraperitoneally three times daily for six days with 6-200 ng purified, bacterially synthesized, murine recombinant GM-CSF. Mice injected with 200 ng rGM-CSF developed a twofold increase in blood neutrophils. In the peritoneal cavity, a dose-related rise was observed in macrophages (up to 15-fold), neutrophils (10-to 100-fold) and eosinophils (10-to 100-fold). Peritoneal macrophages exhibited 15-fold increased mitotic activity (to 7.6/10 (3) cells) and increased phagocytic activity for antibody-coated erythrocytes. Increased numbers of infiltrating neutrophils and monocytes were observed in the liver and lung. Dose-related rises were observed in spleen weight (up to 50%) and the spleen content of monocytes (twofold) and nonerythroid progenitor cells (up to fourfold). A dose-related fall occurred in total marrow cellularity (40%) and total nonerythroid progenitor cells (37%-66%), but levels of neutrophils and monocytes remained constant. The data indicate that the injection of rGM-CSF to normal mice increases overall numbers of granulocytes and macrophages and the phagocytic activity of macrophages and provides direct evidence for the conclusion that GM-CSF is likely to function in vivo as a regulator of these cell populations.