An ELISA for measurement of factor X activation peptide (FXAP) in plasma has been developed. The capture antibody was generated by immunization with a carrier-coupled synthetic peptide based on the amino acid sequence of the C terminal region of native human FXAP: the tag antibody was a commercial polyclonal antibody to factor X. Because of limited specificity of the capture antibody to FXAP compared with factor X, a plasma processing step precipitated plasma factor X and also permitted a concentration step, enabling detection of FXAP below the lower limit of the normal range in plasma. The overall intra- and inter-assay coefficients of variation were approximately 5 percent and approximately 11 percent, respectively. 18 normal laboratory control subjects had FXAP levels of 2.12 plus/minus 0.82 ng/ml (mean plus/minus SEM). Eight patients undergoing surgery and cardiopulmonary bypass progressively generated FXAP throughout the surgery with mean FXAP rising to 11.73 plus/minus 4.66 ng/ml. and this resulted in increased generation of thrombin detected by measurement of plasma levels of F1 + 2. Levels of FXAP rose significantly ahead of those of factor IX activation peptide (FIXAP), supporting a suggestion that contact system activation can not be the primary stimulus to coagulation in bypass. The ELISA to FXAP will be useful in the study of mechanisms of thrombogenesis in clinical situations where the coagulation system is activated.