Proteins fed orally to adult rats are transported into the circulation in the form of break down products of large molecular mass (b.d.p.). As much as 40% of the fed dose, in the case of bovine IgG, is present in the carcase as tungstic precipitable material after so feeding. Ferritin and $\alpha $-gliadin are treated similarly. Preliminary oral immunization with bovine IgG leads to a reduction of this passage if a test dose is given later, but much is still transported. This is in agreement with the expectation that rats which have been fed a normal cereal containing diet should be immune to $\alpha $-gliadin, yet still show transport of this molecule or its b.d.p. The implications of this finding for the physiology and immunology of ingestion are discussed.