SUMMARYIn previous experiments, transplantation antigens were demonstrated on canine vascular endothelial surfaces, suggesting that donor sensitization may take place at the endothelial cell level. This communication describes methods for the collection of canine endothelial cells and the production and characterization of a rabbit anticanine endothelial cell serum. Vascular endothelium was obtained from the aorta of dogs, and the antiserum was raised in New Zealand white rabbits. After appropriate adsorptions with canine erythrocytes and serum protein, γ-globulin was isolated. The resulting isolate showed specific cytotoxic activity for endothelial cells and was not cytotoxic for lymphocytes. The degree of specificity that resulted in these antibody preparations suggests the feasibility of an endothelial cell-masking approach to block vascular sensitization.