Variations in catalase, glutathione peroxidase (GP) and adenylate cyclase (AC) activity in murine erythroleukemic (MEL) cells were studied during multiplication and dimethylsulfoxide (DMSO)-induced differentiation. The results demonstrated that, although DMSO favors the incorporation of 3H-thymidine into DNA of treated cells, it slows down cell multiplication. Increased incorporation was also observed in superoxide dismutase (SOD)-treated cells. DMSO also determined an early and significant drop in AC activity and a late fall in catalase activity, whereas there was no significant variation in GP activity in parallel with the decreased cell multiplication that accompanied cell differentiation. We hypothesize that DMSO and SOD favor 3H-thymidine incorporation by neutralizing the reactive forms of oxygen and that the reduction in catalase and AC activity is closely related to the mitotic activity of MEL cells.