C3H/HeN female mice were exposed to wholebody cobalt-60 radiation and administered soluble glucan (5 mg i.v. at 1 h following exposure), recombinant human granulocyte colony-stimulating factor (G-CSF; 2.5 micrograms/day s.c., days 3-12 following exposure), or both agents. Treatments were evaluated for their ability to enhance hemopoietic regeneration, and to increase survival after radiation-induced myelosuppression. Both glucan and G-CSF enhanced hemopoietic regeneration alone; however, greater effects were observed in mice receiving both agents. For example, on day 17 following a sublethal 6.5-Gy radiation exposure, mice treated with saline, G-CSF, glucan, or both agents, respectively, exhibited 36%, 65%, 50%, and 78% of normal bone marrow cellularity, and 84%, 175%, 152%, and 212% of normal splenic cellularity. At this same time, granulocyte-macrophage colony-forming cell (GM-CFC) values in saline, G-CSF, glucan, or combination-treated mice, respectively, were 9%, 46%, 26%, and 57% of normal bone marrow values, and 57%, 937%, 364%, and 1477% of normal splenic values. Endogenous spleen colony formation was also increased in all treatment groups, with combination-treated mice exhibiting the greatest effects. Likewise, although both glucan and G-CSF alone enhanced survival following an 8-Gy radiation exposure, greatest survival was observed in mice treated with both agents. These studies suggest that glucan, a macrophage activator, can synergize with G-CSF to further accelerate hemopoietic regeneration and increase survival following radiation-induced myelosuppression.