We investigated the effect of protein disulfide isomerase (PDI) on in vivo protein folding of human lysozyme (h-LZM) in a specially constructed yeast coexpression system. Coexpression with PDI increased the amounts of intracellular h-LZM with the native conformation, leading to an increase in h-LZM secretion. The results indicated that PDI is a real catalyst of protein folding in the cell. The secretion of h-LZM increased even when both active sites of PDI were disrupted, suggesting that the effect of PDI resulted from a function other than the formation of disulfide bonds. This is the first finding that PDI without isomerase activity accelerates protein folding in vivo.