These investigations suggest potential host protection for the VLBW infant who is fed fortified human milk. Specific functional benefits, either in host protection, gastrointestinal maturation, or nutrient availability, warrant further investigation. The special properties of human milk have a dual role: to provide nutrients and to provide factors whose concentrations are tailored to the infant's developing capabilities. Not only does human casein, for example, have nutritional importance, but it also modulates calcium and phosphorus concentrations and aids in host defense. Not only do the whey-proteins provide nutritional benefits, but they also are major participants in host defense.