The effects of single subcutaneous injection of cadmium chloride on haemopoiesis in normal (non- irradiated) or irradiated mice were investigated. Cadmium doses used ranged from 1-8 mg/kg body weight. Twenty-four hours after treatment with cadmium (doses from 3 to 8 mg/kg) there were no significant changes in bone marrow cellularity and the granulocyte-macrophage progenitor cell (GM- CFC) number per femur in non-irradiated female ICR mice. Similarly, during the 30-day postinjection period bone marrow cellularity and marrow GM-CFC number in mice treated with a cadmium dose of 5 mg/kg were not significantly different from the control values. Cadmium significantly reduced the lethal effects of gamma rays. In addition, increasing the doses of cadmium administered 24 h prior to sublethal irradiation increased the number of endogenous haemopoietic stem cells (endoCFU-S) in a concentration-dependent manner. Pretreatment with cadmium also decreased the radiation damage to endoCFU-S and haemopoietic progenitor cells committed to granulocyte/macrophage development (GM-CFC). The survival of stem cells was higher and the regeneration of cellularity and GM-CFC of irradiated bone marrow was accelerated in mice pretreated with 5 mg Cd/kg body weight in comparison with saline-injected mice.