Russell's viper (Daboia russelli siamensis) venoms of four different lengths (21-25 cm, 40-60 cm, 81-90 cm and 101-110 cm) were studied by SDS-PAGE electrophoresis and immunoblotting. Studies on biological and biochemical properties of the venom were also carried out. The venom of young snakes (21-60 cm) had a high lethal potency and possessed powerful coagulant and defibrinogenating activities compared to adults (101-110 cm). As snakes aged, these activities decreased. Potent capillary permeability increasing activity was detected in venom of 40-60 cm snakes and was the lowest in the youngest snakes (21-25 cm). SDS-PAGE electrophoresis and immunoblot of venoms showed that venom from the youngest snakes has fewer protein bands; the number of bands increased as the snakes aged. Snakes more than 91 cm long demonstrated no striking differences in the protein pattern.