The macrophage-specific colony-stimulating factor 1 (CSF-1 or M-CSF) is required throughout the G1 phase of the cell cycle to regulate both immediate and delayed early responses necessary for cell proliferation. These are triggered by the binding of the growth factor to the colony-stimulating factor 1 receptor and the activation of its intrinsic tyrosine-specific protein kinase. Phosphorylation of the colony-stimulating factor 1 receptor on specific tyrosine residues enables it to bind directly to cytoplasmic effector proteins, which in turn relay receptor-induced signals through multiple-signal transduction pathways. The activity of p21ras as well as transcription factors of the ets gene family appears to be required for colony-stimulating factor 1 to induce the c-myc gene, and the latter response is essential to ensure cell proliferation. Genes within the fos/jun or activator protein 1 family are targeted via a parallel and independently regulated signal transduction pathway. The continuous requirement for colony-stimulating factor 1 after the immediate early response is initiated indicates that expression of additional delayed early response genes, although contingent on previously induced gene products, might also depend on colony-stimulating factor 1-induced signals. Among the growth factor-regulated delayed early response genes are D-type G1 cyclins, which play an important role in cell-cycle progression.