The investigations reported here were designed to test the effects of a single acute non-lethal treatment of salts of cadmium, selenium and a combination of the two, cadmium selenite, on the time course and extent of erythropoiesis. Peripheral erythrocyte production and splenic erythropoietic activity were used as indices of erythropoiesis. Concerning the latter parameter, the spleen has been shown to be a key organ for erythropoiesis in mice.