Pichinde virus infection of inbred guinea-pigs is a model for arenaviral infections in humans. Infected animals experience reduced levels of multiple coagulation factors caused by either consumption coagulopathy or impaired factor synthesis. A radioimmunoassay (RIA) of guinea-pig fibrinopeptide A (gFPA) has been developed to measure the degree of thrombin action in vivo. gFPA was synthesized via the solid-phase method and conjugated to bovine serum albumin (BSA). A double antibody RIA was established employing goat anti-rabbit IgG to precipitate the primary complex composed of either 125I-5-Tyr-gFPA or 125I-12-Tyr-gFPA and rabbit anti-gFPA-BSA. The cross-reaching material was removed by mixing the plasma with 3 vol of ethanol. The supernatant was filtered through a hollow fibre apparatus by centrifugation. Plasma gFPA immunoreactivities of outbred guinea-pigs averaged 6.56 ng/ml. The gFPA-RIA was validated by determining the quantity of gFPA released from thrombin-degraded fibrinogen. A transient elevation of gFPA levels was detected in Pichinde-infected animals by the gFPA-RIA using 125I-12-Tyr-gFPA as a tracer. The pathogenic mechanism by which the increased gFPA levels may lead to the lethality of Pichinde virus infection remains to be elucidated. It is possible that the coagulopathy triggers changes in immune and inflammatory pathways that induces high cytokine concentrations, with deleterious effects on organs such as the heart and lungs.