Commonly used commercially available plastic catheters were tested and compared for thrombogenicity in dogs. In addition, benzalkonium-heparin complex was applied as a coating to a polyurethane catheter for evaluation. All catheters were thrombogenic when exposed in vivo to arterial dog blood for one hour. The net thrombus weight was determined on an analytical balance. Benzalkonium-heparin coating prevented thrombus formation on polyurethane catheters for one hour. Until a truly nonthrombogenic plastic is found, systemic anticoagulants or heparin-bonding processes appear to be the only available means of preventing thrombus formation in clinical catheterization procedures.