The effect of spingosylphosphorylcholine on cellular proliferation was investigated in a variety of cell types. Spingosylphosphorylcholine at low concentrations greatly stimulated DNA synthesis and cell division in quiescent Swiss 3T3 fibroblasts. The increased DNA synthesis was also accompanied by pronounced morphological alterations. Spingosylphosphorylcholine was remarkably more potent than other known growth factors and also acted synergistically with insulin, epidermal growth factor, fibroblast growth factor, and the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate, to induce cellular proliferation. Sphingosylphosphorylcholine was less effective in stimulating DNA synthesis in rapidly growing normal and transformed cells. Spingosylphosphorylcholine appears to be a new type of potent, wide-spectrum growth promoting agent.