A study was performed on the effect of testosterone on post-irradiation recovery of bone marrow and circulating granulocytes in the rat. Testosterone accelerated recovery of peripheral polymorphonuclear leukocytes, more so if therapy was initiated prior to irradiation. Increased marrow 59Fe compared to controls confirmed the simultaneous erythroid stimulating effect of testosterone.99Tcm-sulfur colloid uptake in the marrow was also noted to be increased in all irradiated groups without a differential testosterone effect, perhaps reflecting a post-irradiation recovery response of increased vascularity. The possible mechanisms are discussed.