An intravenous dose-lethality study of 14 clinically important North and South American pit viper venoms was carried out in mice using standardized, rigorous protocols developed to meet inter­national (WHO) guidelines and US government (FDA) regulations for the development of new treatment products. The study was undertaken in order to establish baseline LD50 values for the ongoing testing of a new crotalid antivenin FAB prepared from sheep immunoglobulin (IgG). The venoms (lyophilized and pooled from juvenile and adult snakes of both sexes) were taken from 10 rattlesnake (nine Crotalus and one Sistrurus) species, two Agkistrodon species, and two Bothrops species. The dose-response data were subjected to probit analysis. Median lethal dose (LD50) values and their 95% confidence limits (CL) were calculated and statistical comparisons between the median potencies were made. The intravenous LD50s (and 95% CL) ranged from a low of 0.13 (0.10-0.15) mg k g 1 for Crotalus durissus terrificus, to a high of 6.32 (5.76-6.94) mg kg-1 for Crotalus horridus horridus. Significant positive rank-order correlations were also found for compar­isons between the present LD50s and published LD50s for intravenous, intraperitoneal, and sub­cutaneous administration.