Lactating rats were given a test solution containing various marker proteins via oral or intravenous routes. Using immunoprecipitation methods for the detection of ovalbumin (OvA), bovine serum albumin (BSA), and bovine gammaglobulins (BIgG) and radioimmunoassay for the analysis of beta-lactoglobulin (beta-LG), these proteins could be found in the blood serum and the milk of the rat 6 h after administration. A specific distribution pattern between the serum and the milk was observed for each protein. The results show that orally fed dietary proteins are able to cross the gut of the lactating rat and can be found in the serum and the milk. It was also observed that oral feeding of the proteins resulted in relatively greater concentrations in the milk than did intravenous administration. This very early presentation to the young of antigenic macromolecules derived from the mother's diet might be of importance for the development of a proper immunological response to common food antigens.