The effect of human interferon (IFN)-beta1b (Betaseron) on the proliferation of cultured human vascular smooth muscle and endothelial cells was tested in vitro. IFN-beta1b inhibited thymidine incorporation and growth of primary cultures of human aortic and coronary artery smooth muscle in a concentration-dependent manner. The same concentrations of IFN-beta1b did not inhibit thymidine incorporation or growth of primary cultures of human aortic or coronary artery endothelial cells. IFN-beta1b induced the expression of MxA (an antiviral protein induced by type I IFNs) in both smooth muscle and endothelial cells, suggesting that both cell types express receptors for type I IFNs. The growth-inhibitory effect of IFN-beta1b could be mimicked by commercially available human IFN-beta, but not by IFN-alpha2 or IFN-alpha8. The effect of IFN-beta1b was species specific, as it did not inhibit thymidine incorporation in aortic smooth muscle cells derived from pig, rabbit, rat, or mouse. The action of IFN-beta1b on smooth muscle cells persisted for at least 4 days following a 24 h preincubation with IFN-beta1b. Human vascular smooth muscle cells treated with IFN-beta1b did not release lactate dehydrogenase, nor did they show any morphologic change, suggesting that IFN-beta1b was not toxic to the human vascular smooth muscle cells. IFN-beta1b inhibited vascular smooth muscle growth while having no growth-inhibitory effect on endothelial cells obtained from the same blood vessel, making it a potential candidate for treating pathologic conditions where abnormal vascular smooth muscle proliferation is implicated, such as restenosis following balloon angioplasty or smooth muscle proliferation following vascular stenting.