In the growth arrested cultures of bovine carotid smooth muscle, K252a (10 - 100 ng/ml), a protein kinase inhibitor with wide spectrum suppressed the cell proliferation induced by TPA and increase of serum. K252a was more potent in the antiproliferative activity than H7, a C-kinase-specific inhibitor, but less than staurosporine, another wide-spectrum protein kinase inhibitor. Since C-kinase plays an important role in the signal transduction leading to the cell proliferation and K252a inhibits C-kinase in vitro, the antiproliferative effect of K252a to carotid smooth muscle cells is likely to be exerted through c-kinase dependent pathway.