[en] Ferritin or tritium labelled immunoglobulin G may, by electron microscopy be demonstrated entering, within and leaving the epithelial cells. Quantitative studies using various proteins labelled with radioiodine show that large amounts of protein bound radioactivity may be demonstrated in the tissues after feeding the labelled protein to adult rats by stomach tube. The molecular size of this material as determined by sugar gradient ultracentrifugation of tissue extracts ranges when IgE is fed from 50,000-20,000 Daltons. The material retains its ability to react as antigen with antisera specific to the original molecule: precipitation reactions may be obtained in gels and quantitative studies show that considerable amounts of the protein-bound radioactivity are still specifically precipitable. Such studies have been carried out with α-gliadin as well as bovine IgG. At 100 days old rats may absorb as much as 40% of a dose of bovine IgG in the form of these large molecular breakdown products. (author)