Various cow milk proteins are used in formulas intended for human infants. However, it is difficult to evaluate which combination of proteins is optimal and also the possible effect of their heat treatment on infant nutrition. The amino acid composition of the proteins does not give adequate information as digestibility of the proteins will determine the extent to which amino acids will be absorbed by the infant. The digestibility will also be affected by the age of the infant. For human infants, the post-prandial amino acid pattern has been shown to reflect protein composition and quality. In this study we used nursing infant rhesus monkeys to evaluate this method. The monkeys were given infant formula based on 6 different protein sources by orogastric intubation. Blood samples were drawn by an in-dwelling catheter at different times post-feeding. Plasma samples were analyzed for free amino acids and blood urea nitrogen (bun). The results show that there is a post-prandial rise and subsequent decline in plasma amino acids and that this varies among the amino acids. Inter-individual variations in response among the infants were relatively large, however, making this assessment difficult in a limited number of animals. Bun values were more consistent among animals and may be used as an indicator of protein digestibility in infants.