In order to study the cell fusion mechanism in chick embryo muscle cells in culture, the changes of cell surface proteins and the effect of FGF during myogenesis were analyzed by using lactoperoxidase-catalyzed iodination and purified FGF addition to the culture medium, respectively. Both quantitative and qualitative changes were observed in $^{131}I$-labelled surface proteins during myogenesis. The proteins with molecular weight of 245K and 66K decreased as the fusion proceeded, and 130K and 93K proteins appeared in the course of fusion. The 245K protein seems to be the LETS protein, and the appearance of 130K and 93K proteins may be due to proteolytic cleavage of the 245K protein. At the same time, four labelled proteins were detected in the cultured medium, which seem to be released from the membrane surface proteins by proteolysis in the course of myoblast fusion. In order to examine the effect of mitogen on myoblast fusion, FGF was partially purified from bovine brain. When FGF was added to culture medium, myoblast exhibited an increase in DNA synthesis and the delay in fusion. These results suggest the possibility that the onset of myoblast fusion is due to the depletion of replication·promoting components in culture medium. The changes of cell surface protein, the engagement of these proteins in myoblast fusion, and the effect of environmental factors on myoblast fusion were discussed.