Relative thrombogenicities of several brands of intravascular catheters were measured in the saphenous and cephalic veins of sheep. Differences in thrombogenicity between brands of up to eleven fold were found. At least one brand contained an ingredient which was locally toxic to sheep and which caused phlebitis in cannulated vessels. The brands tested were made from six types of plastics. It is suggested that differences in manufacturing technique may be more important than the type of plastic in determining the thrombogenicity of catheters.