Colony stimulating factor-1 (CSF-1) and the CSF-1 receptor (the c-fms proto-oncogene product) are expressed during the proliferation of the L6α1 rat myogenic cell line and both are down-regulated during the differentiation to myotubes. Biologically active CSF-1 was shown to be secreted into the culture medium by L6α1 myoblasts and while they could not bind CSF-1, evidence was obtained for cell surface receptor-CSF-1 complexes. It was not possible to block the L6α1 proliferation by incubation with anti-CSF-1 antiserum or suramin. However, in L6α1 myoblasts that were stably transfected with an inducible anti-fms antisense construct, both c-fms protein expression and cell proliferation were more rapidly inhibited under induction and differentiation conditions than parental cells. Furthermore, under these conditions, the c-fms antisense transfected cells also entered myogenic differentiation more rapidly. These results suggest that autocrine regulation by CSF-1 that is intracellular may play a role in the proliferation of muscle cells and that its down-regulation leads to, and may be an obligatory step in, myogenesis.