The young of mother rats which had been immunized against Salmonella pullorum absorbed relatively much less antibody when they suckled during the period shortly after the initial immunization than during the period of hyperimmunity. After hyperimmunization against Brucella abortus both the maternal serum titres attained and the amounts of antibody reaching the young were similar to those of rats given a single immunizing injection of Salmonella pullorum. Antibodies from hyperimmune anti-Salm. pullorum sera produced in a number of species of rodents and in rabbits, and from hyperimmune anti-Brucella abortus sera produced in rats, guinea-pigs and rabbits, when fed to young rats, were absorbed in amounts which were independent of the species in which the antisera were produced but related to the antigen used. The gut of the young rat was therefore showing a selection between antibodies produced in the same animal to the same antigen, the donor differing only in serum titre and length of the immunization period; and between antibodies produced in the same species to different antigens. The fractionation of antisera and titration of the fractions produced evidence that this selection of antibodies is related to their location in the serum proteins.