An additive that is highly effective in small amounts for controlling protein inactivation and aggregation has long been demanded. In this paper we show amidated amino acids as new potent additives. In the presence of 100 mM amidated amino acids, e.g., Ala, Arg, Asn, Met, and Val, the heat-induced inactivation and aggregation of lysozyme at pH 7.1 are one order of magnitude slower than those in the absence of additives. Although a high Arg concentration (> 1 M) has been used to prevent aggregation among amino acids, it is worth mentioning that above amidated amino acids can prevent aggregation at submolar concentrations. The data obtained suggest the importance of amino and amide groups rather than the guanidium group as an aggregation suppressor.