When 125I-labelled rat IgG and 131I-labelled bovine IgG were fed together to 12-days-old rats, they appeared to pass intact into the circulation, the homologous globulin being transmitted preferentially. 3 h after feeding the immunoglobulin mixture, the maximum distribution of radioactivities occured in the distal rather than the proximal portion of the small intestine, and there was a significant change in the ratio of the concentration quotient values ( 125I: 131I) from 0.81 at the pyloric to 1.46 at the distal end of the small intestine. The results of the gel filtration of the luminal contents of the small intestine of fed young rats showed that an effective proteolysis of the administered immunoglobulins occurred within the alimentary tract. The optimum pH for proteolysis was 2.0 for stomach contents, 8.0 for intestinal washes, and 3.4 for intestinal wall extracts.