[3H] Adenine uptake by monolayers of cultured pig aortic endothelium was characterized and used as the basis for a terminal adenine uptake cytotoxicity assay. This assay was used to demonstrate specific cytotoxicity against donor-type endothelium by serum and by peripheral blood lymphocytes from skin-graft allergized pigs, and to compare the time course of these two types of effector mechanism.