Erythroid differentiation of Friend leukemia cells is enhanced when the cells are grown for four days in the presence of dimethylsulfoxide (DMSO). Dimethylformamide (DMF) has a similar though less marked effect. 5‐Bromo‐2′‐deoxyuridine (BUdR) (10−5M) inhibits both DMF‐ and DMSO‐stimulated differentiation. For maximum inhibition, BUdR must be present during the first two days of growth, during which time DNA synthesis is maximal. The addition of BUdR after the third day has no effect. Since BUdR is incorporated into DNA and thymidine prevents BUdR inhibition of DMSO‐stimulated differentiation, it is likely that BUdR acts by virtue of its incorporation into DNA. Although BUdR alone had little effect upon cell multiplication, in combination with DMSO, cell growth was inhibited up to 40%. Since the BUdR‐inhibition of the DMSO effect was approximately 70%, it is unlikely that its effect on differentiation is due to selective killing of those cells which are stimulated to differentiate.