The thermal denaturation of lysozyme dissolved in aqueous phosphate buffer (pH 5.1) and glycerol was studied by Fourier-transform infrared (FTIR) spectroscopy. In both solvents, a single temperature-induced conformational transition was observed but at the distinctly different temperatures of 73 degreesC in aqueous buffer and 94 +/- 2 degreesC in glycerol. No changes in the secondary structure were observed in glycerol up to 90 degreesC. Thus, FTIR data were consistent with the formation of a highly ordered molten globule state at temperatures below 90 degreesC followed by lysozyme unfolding at higher temperatures in glycerol.