Colostral antibodies of cows vaccinated with E. coli B41 (O101: K99, F41) protect completely B41 experimentally infected calves. In order to know more precisely the role of K99, F41 antibodies in protection, calves receiving the colostrum of B41 vaccinated cows are infected with E. coli B44 (O9: K30: H-K99, F41). Vaccination multiplies K99 antibodies in colostrum by seven. In B44 infected calves, specific K99, F41 antibodies protect only 3 out of 6 calves completely. Additive effect of antibodies against K99 and other surface antigens of enterotoxigenic E. coli seems desirable for a more complete protection.