Platelet-activating factor (PAF) stimulates smooth muscle cell (SMC) replication both in vivo and in vitro. In this study we have investigated whether PAF receptor-blocking molecules modulate SMC replication in vitro and the generation of allograft arteriosclerosis in vivo. SMC cultures were established from baby rat aorta media and fibroblast control cultures from the adventitia. Identification of the cultured cell types was determined both by immunohistochemistry and electron microscopy. Both cell types replicated in culture with 10% fetal calf serum (FCS). The addition of PAF-C18 enhanced, and the addition of three PAF receptor inhibitors--WEB 2086, WEB 2170, and BN 50739--reduced, SMC replication and protein synthesis in a dose-dependent fashion in vitro until toxic concentrations were reached. The most potent of these drugs, WEB 2170, was then delivered at the rate of 12 mg/kg per day to recipients of rat aortic allografts. The responses were quantitated by autoradiography after short-term labeling of the recipients with tritium-labeled thymidine (3H-TdR) and by quantitative morphology. Administration of the PAF receptor blocker had no impact on the replication of the inflammatory cells in the allograft adventitia nor on the replication of SMCs in the media and intima. Administration of the PAF receptor blocker delayed the generation of allograft arteriosclerosis slightly, but not significantly. These results suggest that PAF is not an essential component in the inflammatory cascade leading to allograft arteriosclerosis.