The Friend murine erythroleukemia (MEL) cells can be stimulated to differentiate in response to a variety of chemical inducing agents. In the present study, the effect of 1α,25-dihydroxyvitamin D 3 on differentiation of MEL cells was investigated. Vitamin D 3 induced differentiation of MEL cells in culture as determined by elevated hemoglobin content, a rise in the number of benzidine-positive cells and increase in acetylcholine esterase activity. The optimum concentration of the vitamin required to induce differentiation of MEL cells was found to be 750 nM. The pattern of induction of differentiation was similar to that observed with DMSO and the induction of differentiation by vitamin D 3 was inhibited by dexamethasone.