Synthetic vascular grafts have a thrombogenic surface which probably is an important factor when small caliber arterial grafts occlude. We have in the present investigation studied the effect of low molecular weight dextran (Dextran-40, LMWD) in vivo in a sheep model in order to evaluate the graft-surface interaction, using Indium-III-oxine labelled platelets. It was found that the platelet deposition onto polytetrafluoroethylene, PTFE, grafts (4 mm inner diameter), was significantly lower in the dextran treated group than in the control group (p less than 0.001). These differences were also confirmed by histopathological examination where the dextran treated grafts showed a significantly larger thrombus free surface area than corresponding non-treated control grafts. We therefore conclude that low molecular weight dextran has a direct beneficial platelet effect and that the LMWD therefore can be recommended as a justified treatment in connection with small diameter vascular graft reconstructions.