Platelets labeled with a lipid-soluble 111I-8-hydroxyquinoline (oxine) complex were evaluated as agents to measure the rate and degree of in vivo platelet accumulation on catheters. [This study was done because thrombotic and embolic accidents are serious complications of vascular catheterization.] Polyethylene catheters and Teflon-coated guidewires were inserted into the femoral arteries or veins of dogs injected with In-labeled platelets. The catheters were passed to the abdominal aorta or inferior vena cava and left in situ for 30 min-3 h. Images were obtained every 5 min with a .gamma. camera. This labeling technique is reproducible, allows multiple testing with a single preparation and is relatively noninvasive. The results are easily quantitated to allow testing of different materials and coatings.