Introduction of enteral nutrition in early life may lead to autoimmune, inflammatory and allergic reactions. Oxidative damage is a contributing factor in inflammatory bowel disease, necrotizing enterocolitis and ulcerative colitis. Xanthine oxidase (XO) plays an important role in oxidative injuries, because of the generation of free radical hydrogen peroxide (H₂O₂). It can induce oxidative modification and may activate redox-sensitive transcription factors (NF-κB) and proinflammatory cytokines. This study was conducted with the aim to compare the effect of hydrolyzed milk samples, β-casein (CS) and β-casomorphin-7 (CSM-7) on XO activity in culture of fetal human enterocytes (commercial H4 cells). During exposure of H4 cells to hydrolysed milk species, XO activity (U/g prot) decreased significantly treated with hydrolyzates of human colostrum (85.28 ± 7.31*), mature human milk (80.34 ± 5.40*), donkey colostrum (91.82 ± 4.47*), mature donkey milk (71.73 ± 3.89*) vs 165.33 ± 3.40 of control H4 cells. The activity did not change significantly by using cow colostrum (182.20 ± 5.88), but decreased with mature cow milk (101.84 ± 11.56*). Significant decrease was observed for Bebelac formula 0 (96.35 ± 8.89*), but not of Bebelac formula for 12 mo (139.36 ± 5.88). Both, CS and CSM-7decreased XO activity (66.80 ± 6.48* and 67.98 ± 5.32*). Observed activities correlated positively with NF-kB activity, but negatively with proliferative (mitotic activity) of H4 cells. Exclusive breast-feeding has been associated with a reduction in the incidence of chronic inflammatory conditions, including autoimmune diseases, allergies, asthma, atopic dermatitis and inflammatory bowel diseases, what may correlate with decreased oxidative stress. The use of donkey milk seems to be a promising strategy against allergic and other immune-related reactions, regarding oxidative injury and XO activity. Well-documented antioxidative properties of CS may be due to decreased XO activity. Well-documented toxic effects of CSM-7 may not depend on XO activity.