The antitumor compound adriamycin was found to induce differentiation-associated properties in human erythroleukemia K 562 cells. Studies on the relationship between drug concentration and the appearance of hemoglobin demonstrated that the more the rate of cell division was reduced during the 4 days of exposure, the higher was the accumulation of hemoglobin per cell. Considering the variation in the intracellular protein content during the response, it appears that adriamycin (40 nM) stimulated the synthesis of both hemoglobin and non-hemoglobin proteins while hemin, a well-known erythroid inducer, specifically increased hemoglobin synthesis. In addition, the lack of cell growth was associated with a recruitment of benzidine-positive cells. In contrast to hemin, adriamycin did not modify the electrophoretic pattern found in untreated cells.