Chick muscle trophic factor previously reported by us and chick transferrin were studied comparatively in order to clarify their identity. On the basis of physicochemical analyses such as electrophoresis, isoelectric focusing, peptide mapping, absorption spectrum, and iron binding, they were indistinguishable. Also, they were immunologically identical and similarly myogenesis-promoting in vitro. Their trophic activity was likely to be directly dependent on the iron bound. From these results, taken together with those obtained from the comparative study with ovotransferrin, we conclude that muscle trophic factor is most likely identical to transferrin.