A NeuroProbe multiwell microchemotaxis assembly has been used to study the migration of porcine aortic endothelial and smooth muscle cells in vitro in response to serum factors. Whole pig serum was found to be chemokinetic for endothelial cells but not smooth muscle cells. Chemokinetic activity was higher in platelet-enriched, plasma-derived serum (PDS) than in platelet-free PDS. The supernatant from washed, lysed pig platelets also showed chemokinetic activity both for endothelial and smooth muscle cells. Chromatography on Sephadex G-200 showed that the lysate contained a number of activities that act independently on the two cell types. The chemokinetic activity that we have detected appears to be distinct from the chemotactic effect of platelet-derived growth factor and platelet factor 4 on the basis of insensitivity to protamine sulfate and molecular size. The role of these platelet factors in pathological changes in vascular structure is currently under study.