Macrophage-colony stimulating factor (M-CSF) regulates proliferation and differentiation of cells belonging to the monocytic lineage. We investigated the mechanisms of M-CSF differentiation signaling in follicular dendritic cell-P1 cells and analyzed the catalytic activation of different protein kinase C (PKC) isoforms. M-CSF induced rapid catalytic activation of PKC-δ and membrane transocation of the tyrosine phosphorylated form of PKC-δ. Mutation of tyrosine 807 in the M-CSF receptor (Fms) abrogates cell differentiation but not a proliferative response to M-CSF, and FmsY807F failed to activate PKC-δ. We also investigated the downstream signaling pathways from PKC-δ. A cyclic adenosine monophosphate-regulated Ser/Thr kinase gene, protein kinase X (PRKX), has been associated with macrophage differentiation in human cells. We found that M-CSF and PKC-δ induced the expression of the PRKX murine homologue: PKA-related gene. Taken together, our results indicate that PKC-δ functions as a critical mediator of M-CSF-induced differentiation signaling.