Fusion in myogenic cultures is followed by direct microscopic observation and by monitoring the accumulation of creatine phosphokinase (CPK). CPK levels are useful when dealing with cultures in early stages of fusion and in high density cultures, for under these circumstances reliable estimates effusion are impossible to obtain microscopically. The proposition tested is that (i) cell density; (ii) conditioned medium and (iii) cytosine arabinoside can induce precocious fusion in a population of myogenic cells. We find we cannot induce precocious fusion by varying the above exogenous conditions in cells which are not endogenously programmed to fuse. Our data are consistent with the observation that there are two major subpopulations of myogenic cells which, in terms of cell division and fusion, are restricted in their options. There are the replicating presumptive myoblasts which cannot fuse, and the post-mitotic myoblasts which are the only cells in the myogenic lineage that can fuse. These two types of myogenic cells constitute the penultimate and ultimate compartments of the myogenic lineage.