Fms, the macrophage colony-stimulating factor (M-CSF) receptor, is normally expressed in myeloid cells and initiates signals for both growth and development along the monocyte/macrophage lineage. We have examined Fms signal transduction pathways in the murine myeloid progenitor cell line FDC-P1. M-CSF stimulation of FDC-P1 cells expressing exogenous Fms resulted in tyrosine phosphorylation of a variety of cellular proteins in addition to Fms. M-CSF stimulation also resulted in Fms association with two of these tyrosine-phosphorylated proteins, one of which was identified as the 55-kDa Shc, which is shown in other systems to be involved in growth stimulation, and the other was a previously uncharacterized 150-kDa protein (p150). Fms also formed complexes with Grb2 and Sos1, and neither contained phosphotyrosine. Whereas both Grb2 and Sos1 complexed with Fms only after M-CSF stimulation, the amount of Sos1 complexed with Grb2 was not M-CSF dependent. Shc coimmunoprecipitated Sos1, Grb2, and tyrosine-phosphorylated p150, while Grb2 immunoprecipitates contained mainly phosphorylated p150, Fms, Shc, and Sos1. Shc interacted with tyrosine-phosphorylated p150 via its SH2 domain, and the Grb2 SH2 domain likewise bound tyrosine-phosphorylated Fms and p150. Analysis of Fms mutated at each of four tyrosine autophosphorylation sites indicated that none of these sites dramatically affected p150 phosphorylation or its association with Shc and Grb2. M-CSF stimulation of fibroblast cell lines expressing exogenous murine Fms did not phosphorylate p150, and this protein was not detected either in cell lysates or in Grb2 or Shc immunoprecipitates. The p150 protein is not related to known signal transduction molecules and may be myeloid cell specific. These results suggest that M-CSF stimulation of myeloid cells could activate Ras through the nucleotide exchange factor Sos1 by Grb2 binding to either Fms, Shc, or p150 and that Fms signal transduction in myeloid cells differs from that in fibroblasts.