Lysozyme refolding with high yields sometimes results from incomplete denaturation. Dithiothreitol (DTT) is a reductant commonly used to reduce and unfold disulfide-stabilized lysozymes. Through the use of fluorescence spectroscopy to access the extent of denaturation, we found that the rate and extent of denaturation highly depended on the concentration of DTT. Further, the denaturation exhibited a two-phase transition at a high DTT concentration with DTT at > 100 mM and long denaturation time (> 24 h) being needed for complete denaturation. A low DTT concentration and a short denaturation time resulted in fast refolding with high activity recovery, while a high DTT concentration and a long denaturation time resulted in slow refolding with low activity recovery. Hence, the renaturation of disulfide-containing lysozyme was highly affected by the extent of denaturation.