An antithrombogenic aminoethylcellulose coating for Silastic valve prostheses was tested by implantation of coated, heparinized polyethylene cylinders into the thoracic inferior vena cava of dogs. The longest patency rate was achieved in the group that combined aminoethylcellulose (300 mg) with a mixture (2 ml) of Silastic fluid 360 and Silastic RTV 382 in a ratio of 1:2. One cylinder remained patent for 418 days, and the other three had not developed thrombus until 18, 53, and 120 days. In the other study groups, thrombus formation was consistently delayed for one week, but almost all cylinders eventually became occluded. These results are in marked contrast to the control groups where thrombosis had occurred during the first week in ten out of 14 cylinders examined. The aminoethylcellulose coating exerted an anticoagulant effect in vitro although the mechanism was not determined.