Mice carrying null mutations in the myogenic regulatory factors Myf-5 or YyoD have apparently normal skeletal muscle. To address whether these two factors functionally substitute for one another in myogenesis, mice carrying mutant Myf-5 and MyoD genes were interbred. While mice lacking both MyoD and Myf-5 were born alive, they were immobile and died soon after birth. Northern blot and Sl nuclease analyses indicated that Myf-5 (-I-); MyoD (-I-) mice expressed no detectable skeletal muscle-specific mRNAs. Histological examination of these mice revealed a complete absence of skeletal muscle. lmmunohistochemical analysis indicated an absence of desmin-expressing myoblast-like cells. Theseobservations suggest that either Myf-5 or MyoD is required for the determination of skeletal myoblasts, their propagation, or both during embryonic development and indicate that these factors play, at least in part, functionally redundant roles in myogenesis.