Suitable methods for studying the proteins passing into the small intestine are discussed. 2. The proteins passing into temporarily isolated jejunal loops between double re-entrant fistulae in four sheep were studied. 3. Loops about 60-70cm. long secreted protein at a rate of 1-5g./24hr. The effect of slight stimulation of secretion by air pressure on the output of protein in 24hr. was not regular. The total protein in the fluid part of the succus entericus is about 2(1/2) times the serum albumin content of the fluid. 4. The additional protein contained in the cellular debris amounts to about 60% of the protein in the fluid part of the succus entericus. 5. Comparison of the proteins in succus entericus with those in serum by immunoelectrophoretic and other electrophoretic methods showed eight components in the fluid part of the succus entericus that appeared to be the same as those in serum and two components that appeared not to be present in serum. 6. Thin-layer gel chromatography in Sephadex G-200 and sedimentation analysis showed that the succus entericus contained two proteins not present in serum, one with sedimentation coefficient (uncorrected) 10s and one sedimenting slower than albumin: they move with the macroglobulin and slower than albumin respectively on gel chromatography. 7. These proteins could be secreted by the glandular epithelium of the small intestine or liberated from desquamated epithelial cells.
