The cellulolytic enzyme beta-1,4-glucan cellobiohydrolase (CBH) has been isolated from the crude mixture of cellulase enzymes of Trichoderma viride by gel filtration and ion-exchange methods, and some aspects of its kinetic behaviour have been examined. Studies of the initial rates of the CBH-catalyzed production of cellobiose from fibrous alpha-cellulose show that (i) the dissociation constant for cellobiose competitive product inhibition of the reaction is Ki = (1.13 +/- 0.37) X 10(-3) M, (ii) the adsorption of CBH on fibrous alpha-cellulose and its subsequent reaction conform to kinetic equations developed in conjunction with the Langmuir adsorption isotherm, (iii) the rate-pH curve has a maximum at pH 5.2 and decreases at higher and lower pH values, exhibiting enzyme pK values of 3.8 and 6.5, and (iv) the energy of activation of the overall reaction between 5 and 60 degrees C is 5.3 +/- 0.3 kcal mol-1 at pH 5.2. Studies of the time course of the reaction over extended periods of time up to 40% hydrolysis of the cellulose show that (v) the data fit better to a competitive product inhibition model than to models of anticompetitive product inhibition or noncompetitive product inhibition.