Infants who consume casein hydrolysate formula have been shown to have lower neonatal jaundice levels than infants who consume routine formula or breast milk. Because casein hydrolysate has been shown to contain a beta-glucuronidase inhibitor, one possible mechanism to explain this finding is blockage of the enterohepatic circulation of bilirubin by a component of the formula. The aim of this research was to identify the source of the beta-glucuronidase inhibition in hydrolyzed casein. A beta-glucuronidase inhibition assay and measurements of physical and kinetic parameters were used to analyze the components of hydrolyzed casein and infant formulas. Kinetic studies used purified beta-glucuronidase. The L-aspartic acid in hydrolyzed casein accounts for the majority of the beta-glucuronidase inhibition present. Kinetic studies indicate a competitive inhibition mechanism. L-aspartic acid is a newly identified competitive inhibitor of beta-glucuronidase.