On the basis of body weight, mice showed a higher percentage survival to large doses of insulin than did rabbits and these in turn were more resistant to insulin than guinea pigs. Antibodies to insulin were induced in five species of animals. Anti-insulins produced by the guinea pig, rabbit, sheep, and horse can each neutralize insulin extracted from the pancreas of the rabbit, sheep, horse, pig, ox, and monkey (Macacus rhesus), but none of the four can neutralize insulin extracted from guinea-pig pancreas. Guinea-pig anti-insulin can neutralize endogenous mouse insulin; horse anti-insulin cannot. The problem of the relative effectiveness of insulin as an antigen is discussed.