Background:There is limited information in the literature on the capacity of the preterm infant to digest human and bovine milk proteins. We therefore studied in vivo the luminal phase of the hydrolysis of proteins in human milk, human milk fortifier, and preterm formula in preterm rhesus monkeys and in infant rhesus monkeys at 6 weeks and 7 months of age.Methods:Protein hydrolysis was followed by polyacrylamide gradient gel electrophoresis and electroimmunoassay. The serum level of absorbed unhydrolyzed human α-lactalbumin was measured by a radioimmunoassay method. Trypsin and elastase activities in duodenal contents were measured before and after the meal.Results:In 6-week-old monkeys, the enzyme activities decreased by 50% postprandially, whereas they increased in 7-month-old monkeys. In preterm and in 6-week-old monkeys, hydrolysis of human and bovine whey proteins was slow, and in 6-week-old monkeys, 30-50% of the proteins could still be detected immunochemically in duodenal contents after 60 min. At these ages, serum levels of absorbed α-lactalbumin were high. At 7 months of age, no or small (lactoferrin and bovine serum albumin) amounts of the proteins could be detected in duodenal contents after 15 min. At this age α-lactalbumin was not measurable in serum.Conclusion:The low capacity to digest whey proteins in suckling monkeys may depend upon an immaturity of the exocrine pancreas to respond to secretogogues.