Refolding of bovine pancreatic trypsin was carried out. When starting with denatured S-S intact trypsin, the recovered activity attained was 95-100%. In contrast, the recovered activity after refolding denatured S-S reduced trypsin was considerably low compared with other proteases that have been worked with previously. Such low recovered activity was attributed to the small amount of fully reduced trypsin used as starting material for complete refolding. Taking this into account, a recovered activity of 86% could be achieved when using inhibitor-immobilized gels.