During development contractile proteins and the structural elements unique to muscle cells are first detected in the multinucleated cells which form by the fusion of uninucleated myoblasts. Except under unusual circumstances the nuclei of these differentiating muscle cells no longer engage in DNA synthesis. In cell culture the time of initiation of fusion appears to be controlled by some diffusion-mediated processing of the medium which appears also to retard cell proliferation. Although the myoblasts continue to synthesize DNA the distribution of generation times is more variable due, most probably, to protraction of G1. We propose to further examine the nature of the change (or changes) in the medium to determine whether 1.) it can be effected by other cell types as well and 2.) whether its effects can be duplicated by lowering the concentration of known growth-stimulatory medium components. We also plan to examine the thesis that since preparation for fusion and specific biosynthesis appear to occur in G1 (or late G1) the probability of fusion is a function of the length of time spent in G1 (fusion precluding entry into S). The distribution of intervals between M and fusion will be compared with the distribution of G1 times and the temporal sequence established between the decrease in rate of proliferation and the initiation of fusion.