Bioavailability of iron from human milk is exceptionally high. It has been suggested that lactoferrin, the major iron-binding protein in human milk, may participate in this high iron bioavailability from milk. We examined the interaction of lactoferrin with the intestinal brush-border membrane using the rhesus monkey as a model. Brush-border membrane vesicles were prepared from monkeys of various ages. Binding studies with 59Fe-labeled human and monkey lactoferrin were performed to examine interaction of lactoferrin with the brush-border membrane. Specific saturable binding of lactoferrin was found at all ages studied (fetal, suckling infant, weaned infant, juvenile, and adult). The dissociation constant for lactoferrin-receptor binding was 9 X 10(-6) M. In contrast, no binding of serum transferrin or bovine lactoferrin occurred. Removal of fucose from the lactoferrin glycans resulted in a significant decrease in binding. It was concluded that lactoferrin in milk may function in the process of iron absorption through interaction with a small intestinal receptor and that fucosylated glycans on the carbohydrate chain of lactoferrin are necessary for receptor recognition.