Dialyzed serum from clotted monkey blood ("blood serum") promotes the proliferation of monkey arterial smooth muscle cells in culture, but dialyzed serum prepared from recalcified platelet-poor plasma ("plasma serum") is much less effective. Addition of platelets and calcium to platelet-poor plasma increases the activity of plasma serum to the same level achieved with blood serum. Furthermore, addition to plasma serum of a platelet-free supernatant prepared by exposing purified platelets to thrombin also stimulates the proliferation of smooth muscle cells. Thus, much of the growth-promoting activity of dialyzed serum is directly or indirectly derived from platelets. This finding has important implications for the response of arteries to localized injury and provides a key to further understanding of the role of factors derived from blood serum in promoting cell proliferation in vitro.