Endothelial cells provide a continuous nonthrombogenic lining for the vascular tree. Once the endothelium is denuded platelet adhesion and aggregation occur. One postulated mechanism for the nonthrombogenic properties of the endothelial surface is PGI2 production by endothelial cells, which strongly inhibits platelet aggregation. We propose here that the collagen type (s) associated with the endothelial cell surface may also play an important role in these phenomenon. We found that endothelial cells in culture produce types IV and AB2 collagen (both do not aggregate platelets in vitro) and that type AB2 collagen is uniformly distributed on the endothelial cell surface. We have shown this by immunofluorescence microscopy, immunoelectron microscopy, biosynthetic incorporation, and specific immunoprecipitation techniques. Platelets will not aggregate on monolayers of endothelium while they will aggregate on monolayers of other cells cultured from the vascular wall which produce collagen types that aggregate platelets in vitro. Thus, a specific cell surface-associated collagen (type AB2) may be an important determinant in the ability of the endothelial cell to present a nonthrombogenic surface to the blood.