We investigated the effect of activin A on the In Vitro differentiation of primary myogenic cells isolated from chick embryonic breast muscle. As judged by the morphology of the cells and accumulation of creatine kinase, activin A inhibited myogenesis in a dose-dependent manner. The inhibitory activity was reversibly suppressed by follistatin. Activin A also inhibited the expression of MyoD1. Our data also suggest that the inhibitory activity of activin A is exerted neither on proliferation of myoblasts or the growth of myotubes, but rather an early phase of differentiation following the withdrawal from cell cycle. A comparison of actions of activin A and TGF-β suggests that points of action is similar.