A novel reaction of cytochrome-c from the horse heart with the enzyme glucose oxidase from Aspergillus niger (EC 1.1.3.4), in acidic media is described. Glucose oxidase is able to induce a rapid, profound and irreversible physico-chemical change in cytochrome-c, under anaerobic conditions and in the presence of glucose. The initial rate of reaction is almost independent of the concentration of enzyme and glucose. The striking feature of this reaction is the fact that the reaction proceeds efficiently even below a concentration of 10 nM enzyme.