The acute thrombogenicity of a new polyurethane urea graft has been compared with polytetrafluoroethylene (PTFE) grafts using labelled homologous platelets in the carotid arteries of sheep in a low-flow model. There was no difference in patency. Thrombus weight and thrombus free surface were the same in both types of graft after 4 h. The accumulation of homologous 32P platelets was measured at the 2 anastomoses and at a midgraft position. At no point was there any statistically significant difference between the 2 grafts, but the activity was always somewhat higher in the polyurethane grafts. Although more compliant, in this study polyurethane urea grafts behaved much like PTFE ones.