An immunizing mixture comprising a small number of rattlesnake venoms produces an antivenin with neutralizing power extending rather widely to venoms of other species of pit viper. Venoms of North American pit vipers form three antigenic groups, as follows: (a) most of the rattlesnake venoms; (b) venoms capable of causing late paralytic symptoms; (c) moccasin venoms. To yield a satisfactory polyvalent antivenin at least one venom from each of these groups must be included in the mixture used for injection of the antivenin-producing animal.