Centrifugation in density gradients was used to study the fragments produced during intraluminal and intracellular digestion, after the injection of 125I-labelled immunoglobulin G (IgG) into different regions of the small intestine of 14 to 15-day-old (pre-closure) and 24-day-old (post-closure/ rats. 2. After injection into the proximal small intestine and into the ileum of pre-closure animals, the bulk of the radioactivity recorded for gut washes and gut homogenates was located at 4S-7S. The serum from animals which had received injections into the proximal small intestine had high radioactivity and one peak at 7S; the serum from animals which had received injections into the ileum had low radioactivity and no activity in the 7S region. 3. After injection into the proximal small intestine of post-closure animals, the bulk of the radioactivity recorded for gut wash samples was located at 3-5S--5S. Gut homogenates had peak activity at 2-5S--4S. Thus large molecular weight products can be absorbed by the proximal enterocytes of post-closure rats and degraded. The sera of these animals had low radioactivity. 4. After injection into the distal small intestine of post-closure animals, the bulk of the radioactivity recorded for gut wash and gut homogenate samples was located at 4S-7S and in this respect the radioactivity plots resembled those for (2) above. Serum radioactivity was low. 5. The effect of precipitation with trichloroacetic acid and incubation with specific antiserum upon the radioactivity of gut washes, gut homogenates and serum samples was recorded. 6. The relevance of these findings to studies on the transmission of protein by the rat small intestine is discussed.
