A radioimmunoassay for measurement of canine fibrinopeptide A, which provides a quantitative index of thrombin action on canine fibrinogen, was applied to measurements of the thrombogenicity of angiographic guide wires. Benzalkonium-heparin-treated Teflon-coated, untreated Teflon-coated, and uncoated stainless steel wires were compared in an experiment in dogs which closely simulated the use of these wires in humans. During the first 4 min after insertion, the benzalkonium-heparin-treated Teflon-coated wire was associated with significantly greater systemic levels of fibrinopeptide A than were either Teflon-coated or stainless steel wires. Beyond 10 min, markedly increased levels of fibrinopeptide A were measured in association with these latter two wires compared to the benzalkonium-heparin-treated Teflon-coated wire. Both the early rises in fibrinopeptide A associated with the benzalkonium-heparin-treated Teflon-coated wire and the late increases in fibrinopeptide A associated with the Teflon-coated and uncoated stainless steel wires could be eliminated by prior systemic administration of low dose heparin (40U/kg). These studies suggest that, at least in regard to activation of the plasma clotting system, the benzalkonium-heparin treatment offered no advantages over the other wires tested in terms of reduced thrombogenicity within the critical 10 min following insertion. No difference could be discerned between uncoated stainless steel wires and Teflon-coated stainless steel guide wires in this test.