Experimental conditions were established whereby it was possible to control the extent of reduction of the disulfide bonds of the Bowman-Birk soybean inhibitor (BBI) with sodium borohydride. Losses in the antitryptic and antichymotryptic activities of BBI were directly dependent on the average number of disulfide bonds which had been reduced. Both activities were completely lost upon the reduction of an average of four out of the seven disulfide bonds of BBI. Disc gel electrophoresis of partially reduced, carboxymethylated preparations of BBI revealed a complex mixture of the native and fully reduced protein as well as partially reduced intermediates present in varying proportions depending on the state of reduction. It was possible to regenerate the activity of a solution of fully reduced BBI by dilution with a disulfide–thiol redox buffer similar to that which had been employed by Saxena and Wetlaufer (Biochemistry, 9, 5015, 1970) for the reactivation of reduced lysozyme. It was concluded that BBI probably has four reducible disulfide bonds which are essential for inhibitory activity against trypsin as well as chymotrypsin.