Five derivatives of lysozyme which were modified in four of the original six lysine residues were prepared by reductive alkylation with three aliphatic aldehydes of different chain lengths and two hydroxyaldehydes of different hydroxyl-group contents. The thermal stability of these derivatives was investigated by DSC. The alkylated derivatives were less stable than unmodified lysozyme, depending on the size of the introduced alkyl groups. However, there was no significant difference in the thermal stability among unmodified and two hydroxylated lysozymes, The change in the thermal stability upon lysine modification of lysozyme is remarkably dependent on the qualitative factors of the entering substituents, and on other factors such as steric hindrance.