Protein disulphide isomerase (PDI) shows chaperone and anti-chaperone activities in assisting refolding of denatured and reduced lysozyme in redox Hepes buffer, but only chaperone activity in phosphate buffer and redox Hepes buffer containing 0.1 M NaCl. In non-redox Hepes buffer its anti-chaperone activity is very weak. PDI displays its anti-chaperone activity only for those substrates showing relatively low aggregation during refolding, and is strongly dependent on refolding conditions, of which ionic strength appears to be an important factor. The S-methylated PDI, fully active as a chaperone but devoid of isomerase activity, by itself shows only anti-chaperone activity, but reinforces rather than suppresses the chaperone activity of native PDI in the refolding of lysozyme. A fragment of PDI with the C-terminal peptide-binding sequence removed and devoid of chaperone activity does not show anti-chaperone activity in lysozyme refolding. It appears that the anti-chaperone activity of PDI is dependent on its chaperone activity.