The growth regulatory protein oncostatin M was initially discovered in macrophage-conditioned medium. We investigated the effects of oncostatin M on cultured rabbit aorta smooth muscle cells (SMCs) and found that the peptide stimulated an increase in the incorporation of [3H]thymidine into DNA. The magnitude of the stimulation was dependent on oncostatin M concentration and SMC confluency. In subconfluent cultures, 1-2 nM stimulated 4- to 5-fold increases in DNA synthesis after 20 hr. Other structurally related cytokines (granulocyte colony-stimulating factor, leukemia inhibitory factor, interleukin 6, ciliary neurotrophic factor) did not affect SMC DNA synthesis. After 5 or 8 days, oncostatin M caused a doubling in SMC number and also induced a transformed phenotype. The combination of oncostatin M and platelet-derived growth factor for 8 days resulted in a 4-fold increase in cell number, approximately the same increase in cell number as induced by the addition of 10% fetal calf serum. Further investigation suggested that the mitogenic effect of oncostatin M was in part due to tyrosine kinase activation. Within 1-2 min, the factor increased phosphotyrosine levels of several SMC proteins. In addition, detectable increases in diacylglycerol levels occurred within 2-5 min, reached 50% above control by 30 min, and remained elevated through 45 min of incubation with oncostatin M. SMC inositol phosphate levels were also elevated within 2 min and then returned to near control values by 20 min. Within 30 min, oncostatin M induced expression of the immediate-early gene EGR-1. These data indicate that oncostatin M may be an important, naturally occurring mitogen for vascular SMCs.