The function of mos protein in somatic cells, the mechanisms underlying its ability to transform cells, and its relationship to growth factor autonomy and growth factor-mediated signal transduction are not well defined. This report demonstrates that the expression of transforming mos (v-mos) can block the stimulation of growth-related gene expression mediated by the platelet-derived growth factor (PDGF-BB). This blockade by v-mos of PDGF-BB signal transduction occurs very early in the signalling pathway, at the level of PDGF type-beta receptor autophosphorylation. Although the expression of PDGF type-beta receptor, as detected by Western blot with anti-PDGF type-beta receptor antibody,was not diminished in v-mos transformed BALB/c-3T3 murine fibroblasts, the autophosphorylation of PDGF-beta receptor in response to ligand (recombinant PDGF-BB homodimer) stimulation was profoundly suppressed. This same phenomenon of v-mos-mediated PDGF type-beta receptor autophosphorylation inhibition was also demonstrated in NIH-3T3 fibroblasts. A v-mos mutant gene, which was incapable of binding ATP and was kinase-defective, did not block ligand-mediated receptor autophosphorylation. Factor(s) present in v-mos expressing fibroblasts, and found in the membrane fractions of these cells, dominantly inhibit the autophosphorylation of the PDGF type-beta receptor obtained from normal fibroblasts. This trans-acting factor does not appear to be a protein-tyrosine phosphatase. These findings suggest a role for mos, or a similar serine/threonine kinase, as a control mechanism in one of the earliest steps of the PDGF signal transduction pathway, and may provide a model for the functional interaction of mos with growth factor receptors.