Interactions of vascular endothelial cells (ECs) and smooth muscle cells (SMCs) were studied by testing the ability of cultured bovine aortic ECs to secrete factors influencing the migration of cultured aortic SMCs from the same species. Migration of SMCs was examined in blind-well chambers using gelatin-coated polycarbonate filters. Conditioned culture medium obtained by incubating confluent monolayers of ECs in serum-free RPMI-1640 medium for 48 hours caused a 2.4-fold increase in the migration of SMCs as compared with nonconditioned medium (p less than 0.001). The effect was dependent on the length of conditioning with the ECs and was chemotactic in nature as judged on the basis of checkerboard analysis. Preliminary characterization of the migration stimulating activity indicates that it is sensitive to trypsin, nondialyzable, and stable at 56 degrees C for 30 min. The activity was abolished by heating to 100 degrees C for 20 min but was not significantly inhibited by protamine sulphate, which suggests that most of the activity was not due to platelet-derived growth factor (PDGF)-like proteins. Our results thus show that ECs secrete polypeptide(s) chemotactic for vascular SMCs. Such interactions between ECs and SMCs in vivo might contribute to the migration of medial SMCs into the intima during atherogenesis.