Adsorption of bovine fibrinogen from dilute plasma and human serum albumin (HSA) from buffered HSA solution to low-temperature isotropic (LTI) pyrolytic carbon (Pyrolite) and polyetherurethane urea (Biomer) was measured. Whereas fibrinogen adsorption to Biomer passed through a maximum at intermediate plasma dilution(a typical Vroman peak), maximal adsorption to Pyrolite occurred from undiluted plasma; i.e., there was no Vroman effect. Protein elution by sodium dodecyl sulfate (SDS) surfactant was also measured. The amount of fibrinogen eluted from both substrates by SDS was less when the protein was adsorbed from more dilute plasma or eluted after 4 days postadsorptive residence time. Under all conditions, fibrinogen retention by Pyrolite was greater than that by Biomer. Binding of a monoclonal anti-fibrinogen (against residues 402-411 of the gamma chain) to substrates preadsorbed with human plasma was measured using an ELISA. Antibody binding (normalized to the amount of fibrinogen adsorbed) to plasma preadsorbed Pyrolite was much less than that to preadsorbed Biomer. On both surfaces, normalized antibody binding was much greater when fibrinogen adsorption had occurred from more dilute plasma. Monolayer adsorption of HSA to Pyrolite was observed. HSA retention was greater when the protein was adsorbed from more dilute solution. These results demonstrate that proteins are tenaciously bound to Pyrolite compared with Biomer. Therefore, the thromboresistance of Pyrolite may be partly due to the lowered reactivity of the platelet binding domain, as well as a lessened ability of tightly bound fibrinogen to interact with platelets.