In multinucleated heterokaryons formed from the fusion of differentiated muscle cells to either hepatocytes or fibroblasts, muscle-specific gene expression is activated, liver-specific gene expression is repressed, and there are changes in the location of the Golgi apparatus. An understanding of the regulatory mechanisms that underlie this plasticity is of particular interest given the stability of the differentiated state in vivo. We have now investigated whether MyoD or myogenin, regulators of muscle-specific gene expression that have a helix-loop-helix motif, can induce the phenotypic conversion observed in heterokaryons. When these regulators were stably or transiently introduced into fibroblasts or hepatocytes by microinjection, transfection or retroviral infection with complementary DNA in expression vectors, fibroblasts expressed muscle-specific genes, whereas hepatocytes did not. However, fusion of hepatocytes stably expressing MyoD to fibroblasts resulted in activation in the heterokaryon of muscle-specific genes of both cell types. These results imply that other regulators, present in fibroblasts but not in hepatocytes, are necessary for the activation of muscle-specific genes, and indicate that the differentiated state of a cell is dictated by its history and a dynamic interaction among the proteins that it contains.