The role of complement activation in thrombogenesis was investigated on the surface of hydrophilic monomer-graft copolymerized polyethylene (PE) tubes. N-vinylpyrrolidone (NVP)-grafted tubes activated in an in vitro complement system of canine serum; but no activation occurred in 2-hydroxyethyl methacrylate (HEMA)-grafted tubes. The relative patent time for NVP-grafted tubes implanted in canine peripheral veins was shorter than that for HEMA-grafted tubes and adhesion of numerous leucocytes was observed on the luminal surfaces of the NVP-grafted tubes. Decomplementation by prior administration of cobra venom factor elongated the relative patent time for NVP-grafted tubes only and also inhibited the adhesion of leucocytes onto them. These results suggest that the complement activation participates in thrombus formation on the polymer surfaces in canine veins.