Following the report of a WHO Coordination Meeting on the biological characteri› zation of medically important venoms and standardization of antivenoms, methods have been developed for the assessment of lethal defibrinogenating, procoagu1ant, haemorrhagic and necrotizing properties of a wide range of venoms of medical importance throughout the world. The venoms studied included Echis carinatus (Iran), Naja naja kaouthia (Thailand), Notechis scutatus (Australia), Trimeresurus f1avoviridis (Japan), Vipera russe11i (Thailand) and Crotalus atrox (USA) which include six of the eight venoms designated as the proposed international standards for venoms (PSiS). (C. atrox venom replaced C. adamanteus venom as an adequate supply of the latter was not available.) Forty-seven other venoms have also been studied. The tests for estimating these biological activities are simple and should be reproducible in other laboratories throughout the world. Procedures for assaying neuromuscular paralytic activity of 