The erythroid and granulocytic repopulating ability of rat bone marrow has been measured after daily administration of three alkylating agents: dimethylmyleran, busulphan and cyclophosphamide. Three different patterns of response were observed. The results emphasize the variability of the response of haemopoietic tissue to repeated administration of different cytotoxic drugs and provide further evidence that erythroid and granulocytic repopulating assays measure different populations of precursor cells.