Twoin vitrointestinal models were used to investigate postnatal maturational changes of the gut barrier functions. Microvillus membrane (MVM) preparations were studied for surface binding, and everted gut sacs were studied for mucous coat binding, breakdown and uptake of radioiodinated bovine serum albumin (BSA), and β-lactoglobulin (β-LG). Surface binding of these proteins to MVM was weak and nonspecific. There was more binding of both proteins to immature MVM (BSA: newborns, 2.74 ± 0.52%, adults, 1.08 ± 0.17%, p<0.001; β LG: newborns, 6.30 ± 0.54%; adults, 2.05 ± 0.07, p<0.001). In contrast to MVM binding characteristics, mu-cous coat binding of the cow's milk proteins to immature gut sacs was significantly less (BSA: preweanlings, 0.94 ± 0.30 µg125-I-protein Eq/mg mucosal protein; adults, 3.06 ± 0.74, p<0.001; β-LG: preweanlings, 5.61 ± 1.48; adults, 9.83 ± 1.33, p<0.001). Protein binding and uptake were correlated in the immature animals (r=0.76, p<0.001 for BSA and r=0.85, p<0.001 for β-LG). More β-LG was bound and taken up than BSA in the preweanlings (p<0.001). Trichloroacetic acid precipitation studies showed that, even in the immature rats, β-LG was much more readily broken down by mucosa-associated enzymes than BSA. Immature animals showed less protein breakdown than adult controls. Decreased protein breakdown and mucous coat binding as well as increased MVM binding may account for the increased uptake of intact food antigens in the newborn. Differences between increased MVM binding and decreased mucous coat binding of cow's milk proteins are attributed to protective elements such as the mucus layer which is present in the gut sac model but lacking in MVM.