Two mutants of human lysozyme were synthesized. Mutant A92D, in which Ala92 was substituted by Asp, contains a partial Ca2+-binding site and mutant M4, in which Ala83, Gm86, Asn88 and Ala92 were replaced by Lys, Asp, Asp and Asp respectively, contains the complete Ca -binding site of bovine a-lactalbumin. The Ca2+-binding constants of wild type human lysozyme and of mutants A92D and M4, measured at 25°C and pH 7.5, were 2(±1) x 102 M“1, 8(±2) x l^M“1 and 9(±0.5) x 10* M”1 respectively. Information gathered from mkrocalorimetrk and CD spectro-scopic measurements indicates that the conformational changes of the M4 mutant lysozyme, induced by Ca2+ binding, are smaller than those observed for bovine a-lactalbumin and for the Ca2+-binding equine lysozyme. At pH 4.5, the thermostability of both the apo and Ca2+ forms of the A92D human was decreased in comparison with that of native human lysozyme. In particular, within the apo form of this mutant an a-helix-containing sequence was destabilized. In contrast, at the same pH the thermostability of the apo and Ca2+ forms of the M4 mutant lysozyme was increased. The e-ammonium group of the Lys83 side chain is assumed to be responsible for the stabilization of the apo form of this mutant.