The dimethyl sulfoxide induced erythropoietic differentiation of murine erythroleukemia cells, as determined by scoring benzidine positive cells, is inhibited by mitomycin C at concentrations that have no effect on cell proliferation. The inhibition occurs only when cells are treated with mitomycin C during induction and has a limit value of about 50%, independent of mitomycin C concentration. This limit value does not depend on cell heterogeneity since genetically homogeneous subclones, derived from DS19 clone, show levels of mitomycin C inhibition between 16 and 50%. Treatment with mitomycin C at different times after dimethyl sulfoxide addition shows that cell sensitivity to inhibition is not homogeneous during the induction period; it is maximal between 18 and 24 h from the start of induction and is observed with a concentration of mitomycin C as low as 25 fM. The inhibition of the benzidine positive phenotypic expression appears irreversible since this effect is observed on cells even several generations after those which were actually treated.