We have recently reported that the intestinal transport of intact macromolecules into the circulation decreases with age presumably due to maturation of mucosal barrier factors. To extend this observation and determine the effect of natural versus artificial feeding on maturation of intestinal mucosal “barrier function,” we conducted experiments which assessed both macromolecular transport and epithelial cell morphology.To study barrier function, we gavage fed a physiologic quantity (100 mg) of bovine serum albumin (BSA) to weight-matched breast- and bottle-fed infant rabbits at 1 and 2 wk of age and quantitated intestinal macromolecular transport by measuring circulating plasma concentrations of the intact antigen 4 hr later. A significant decrease (P< 0.02) in immunoreactive bovine serum albumin (1-BSA) concentration was noted in breast-fed (6.12 × 0.77 μg I-BSA per ml plasma) compared with bottle-fed (9.19 × 0.93 μg I-BSA per ml plasma) animals at one wk. However, at 2 wk, no difference could be demonstrated between the two groups. Furthermore, small intestinal morphology evaluated by light and electron microscopy was similar in both groups at each age.To determine if the lower plasma I-BSA noted at one wk in naturally fed animals was related to the presence of anti-BSA antibodies in breast milk and/or in plasma of the pups, breast milk and plasma from the breast-fed animals was evaluated by counter-immunoelectrophoresis and hemagglutination. No anti-BSA antibodies were detected. Moreover, plasma from breast- and artificially fed rabbits not gavage fed BSA contained no I-BSA.These data suggest that intestinal transport of antigens in the immediate neonatal period is decreased earlier in breast- as compared to bottle-fed animals. Therefore, we suggest that breast milk may exert a protective function to control the transport of potentially antigenic molecules into the systemic circulation of newborn animals by either facilitating the early maturation of intestinal barrier function or by providing passive barrier factors until the newborn's natural barrier can develop.