Small potent inhibitors of aggregation are eagerly demanded for preventing the inactivation of proteins. This paper shows that amino acid esters (AAEs) prevent heatinduced aggregation and inactivation of hen egg lysozyme. Lysozyme was completely inactivated (< 1% original activity) during heat treatment at 98 degrees C for 30 min in a solution containing 0.2 mg/mL lysozyme in 50 mM Na-phosphate buffer (pH 6.5). The residual activities only slightly increased (< 5%) in the presence of 100 mM commonly used additives such as arginine, guanidine, urea, and sugars. However, in the presence of 100 mM AAEs, the residual activities were > 60% and no aggregates were observed during the heat treatment at 98 degrees C for 30 min. This fact provides new information on the scaffold for designing additives to prevent heat-induced aggregation.