The action of a fibrin-promoting enzyme isolated from the venom of A.c.contortrix was investigated. The ratio of fibrinopeptides A and B released was similar in isolated human and rat fibrinogen and human plasma, fibrinopeptide B always being released preferentially. However, no clotting occurred in rat plasma, and no fibrinopeptides were released even after prolonged incubation. The results suggest strong, fast-acting irreversible neutralization of the enzyme activity in rat plasma.