A selective procedure using synthetic substrates for determination of exo-1,4,-β-glucanases in a mixture of exoglucanases, endoglucanases, and β-glucosidases is formulated. The heterobiosides, p-nithrophenyl-β-d-cellobioside (pNPC) or p-nitrophenyl-β-d-lactoside (pNPL), were used as selective substrates for the measurement of exoglucanase activity. The exoglucanases (especially cellobiohydrolases, which split off cellobiose units from the nonreducing end of the cellulose chain) specifically act on the agluconic bond (between p-nitrophenyl and the disaccharide moiety) and not on the holosidic bond (between the two glucose units of cellobiose). The interfering effect of β-glucosidase, which acts on both agluconic and holosidic bonds, is overcome by the addition of d-glucono-1,5-δ-lactone, a specific inhibitor of β-glucosidases. The interference of endoglucanases, which also act on both agluconic and holosidic bonds, can be compensated for by prior standardization of the assay procedure with a purified endoglucanase from the studied mixture of cellulases.