Gliclazide(hypoglycemic drug having sulfonylurea structure)-degrading activity was found in fraction M(macroglobulin, Fr. M) obtained from pooled human serum by gel filtration using a Sephadex G-150 column. The main degrading activity was in the fraction eluted from the Fr. M-subjected DEAE-cellulose column with 0.4 M phosphate buffer (pH 5.2), and the gliclazide-degrading protein localized around alpha 2 to beta-globulin on an electrophoretic pattern using a cellulose acetate membrane. The degrading activity was enhanced about two-fold by lyophilizing Fr. M solution containing a higher sodium phosphate (Na2HPO4-NaH2PO4), over 0.27 M. This indicates that the appearance and enhancement of the degrading activity required the combination of the lyophilization of the sample solution and a certain initial concentration of sodium phosphate prior to lyophilization.