Regulation of the transition of mouse myoblasts from proliferation to terminal differentiation was studied with clonal density cultures of a permanent clonal myoblast cell line. In medium lacking mitogenic activity, mouse myoblasts withdraw from the cell cycle, elaborate muscle-specific gene products, and fuse to form multinucleated myotubes. Addition of a purified mitogen, fibroblast growth factor, to mitogen-depleted medium stimulates continued proliferation and prevents terminal differentiation. When mitogens are removed for increasing durations and then refed, mouse myoblasts irreversibly commit to terminal differentiation: after 2-4 h in the absence of mitogens, myoblasts withdraw from the cell cycle, elaborate muscle-specific gene products, fuse in the presence of mitogens that have been fed back. Population kinetics of commitment determined with 3H-thymidine labeling and autoradiography suggests the following cell-cycle model for mouse myoblast commitment: 1)if mitogens are present in the extracellular environment of myoblasts in G1 of the cell cycle, the cells enter S and continue through another cell cycle; 2) if mitogens have been absent for 2 or more hours, cells in G1 do not enter S; the cells commit to differentiate, permanently withdraw from the cell cycle (will not enter S if mitogens are refed), and they subsequently elaborate acetylcholine receptors and fuse (even if mitogens are refed); 3) cells in other phases of the cell cycle continue to transit the cell cycle in the absence of mitogens until reaching the next G1. the commitment kinetics and experiments with mitotically synchronized cells suggest that the commitment "decision" is made during G1. Present results do not, however, exclude commitment of some cells in other phases of the cell cycle.