Summary: With American pit viper venoms the fatal dose for 20 gram mice injected intraperitoneally approximates that for 350 gram pigeons injected intravenously. No difference in time of onset and character of paralysis from very toxic and less toxic venoms, such as was observed in pigeons, was seen in mice. Venoms causing late paralysis and showing better neutralization by “Cascabel” antivenin in the pigeon are also better neutralized by this antivenin in the mouse. Certain bothropic venoms are equally well neutralized by the two types of antivenin. Certain foreign true viper venoms are neutralized to some extent by both types of antivenin, others little if at all.