An animal study was undertaken to investigate possible prophylactic use of a silicone coating in vivo. Catheters, stainless steel and teflonized guidewires were either coated with silicone on heparin-quaternary-ammonium compounds. Uncoated guidewires and catheters were used as controls. The heparinized surfaces showed no fibrin deposit, and the siliconized surfaces showed at least as much fibrin as the controls. It was also noted that the fibrin was stripped easily from the siliconized catheters. It is concluded from this study that silicone compounds may well reduce the Lee-White clotting time in vitro, but the thrombogenicity of foreign surfaces is basically unaltered in vivo. Quaternary ammoniumheparin surfaces were shown to give a reliable short term protection from thrombus formation.