This study was designed to investigate the prevention of fibrin deposition on the external surface of catheters. Pairs of catheters were introduced percutaneously into both femoral arteries of dogs, and fibrin deposition on the surface of the catheters was observed angiographically, one of the catheters serving as control. The systemic application of heparin in dosages below the full heparinization dose prevented fibrin deposition. On the other hand, dextran, which is said to decrease platelet adhesiveness, proved ineffective even if used in maximal dosage. A simple technique was developed to form insoluble precipitates between heparin and quaternary ammonium compounds. These precipitates are soluble in various organic solvents and form a waxy, water insoluble coating upon evaporation of the solvent. Simple surface coating of various catheter materials appeared to delay fibrin deposition on the outside of the catheters for several hours. The benzalkonium chloride-heparin precipitate proved to be of very low toxicity as tested in mice. The advantage of the described technique is its simplicity since it can be easily applied to surfaces such as plastics, metal, and glass by coating, eliminating the previously described complex heparin-binding procedures.