Scintillation camera imaging of 111Indium-labeled platelets has been used to measure acute thrombus formation on modified expanded Teflon (ePTFE) vascular grafts (Gore-Tex, 4.0 mm i.d. x 3-5 cm in length, W. L. Gore and Associates, Inc., Flagstaff, AZ) placed in the carotid arteries of normal baboons. Platelet deposition plateaued over 2 hr postoperatively and occurred primarily at the graft-vessel anastomoses. A positive correlation was found between the circulating platelet count in individual animals and the extent of early platelet thrombus deposition. Unmodified ePTFE grafts accumulated 4.6 +/- 1.2 x 10(9) platelets per graft, or 2.3 +/- 0.71 x 10(9) platelets per anastomosis. Acutely, platelet accumulation was reduced versus control graft results by coating the graft lumenal surfaces with a smooth layer of silicone rubber polymer (0.60 +/- 0.19 x 10(9) platelets per anastomosis; P less than 0.02) but not by coating the grafts using a plasma polymer based on methane, which did not modify graft texture (8.2 +/- 1.7 x 10(9) platelets per graft; P greater than 0.10). The benefit of the silicone rubber coating persisted for at least 48 hr. However, longer term patency was not preserved because 10 of 12 grafts placed had failed within 1 to 2 months.