Serial venom antigen levels were measured by enzyme-linked immunosorbent assay (ELISA) in 46 patients with systemic envenoming by the Malayan pit viper (Calloselasma rhodostoma), a major cause of snake bite in Southeast Asia. The principal effects of the venom are defibrination, hemorrhage and local tissue necrosis. Admission venom levels, which varied between 0 and 595 ng/ml, correlated with the incidence of spontaneous systemic bleeding, blood incoagulability and concentrations of plasma fibrinogen and serum fibrin degradation products. The presence or absence of nonclotting blood also correlated with the time elapsed between the bite and hospital admission. The development of nonclotting blood may be delayed by up to 72 hr after the bite even though circulating venom and raised FDP may be detected at presentation. This is probably explained by a temporary equilibrium between synthesis and consumption of fibrinogen. Venom antigenemia recurred in 12 patients (26%) suggesting continuous absorption of venom from the wound or saturation of extravascular binding sites. Admission venom levels also correlated with the extent of local swelling and the occurrence of tissue necrosis at the site of the bite. Venom was detected in 87% of wound aspirates and 88% of urine specimens taken on admission. Tourniquets, of the type used in rural Thailand, did not delay the absorption of venom into the circulation.