The percent in-vitro digestibility of proteins in human milk was determined by three different methods and compared with the in-vitro digestibility of proteins in Enfamil, Similac and Isomil. In-vitro digestibility was determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), pH drop and a simultaneous dialysis method. Significant differences (P<0.05) were observed in the in-vitro protein digestibility of human milk and the three infant formulas. Estimated percent in-vitro protein digestibility of each of the samples was also found to be significantly affected by the method of determination. The in-vitro protein digestibility of all four samples estimated by simultaneous dialysis was lower than results obtained by SDS-PAGE and pH drop methods. Except for Isomil, results obtained by the pH drop method were lower than those determined by SDS-PAGE. The in-vitro protein digestibility of Enfamil and Similac was found to be greater than that of human milk by all three methods. Except for results obtained by SDS-PAGE, the in-vitro digestibility of proteins in Isomil was found to be greater than for the proteins in human milk. These results indicate that the proteins in human milk are not as extensively hydrolyzed in-vitro as the proteins in powdered infant formulas. / Master of Science