1. Four principal endoglucanase components of Trichoderma koningii cellulase were separated and purified by gel filtration on Sephadex G-75, ion-exchange chromatography on DEAE- and sulphoethyl-Sephadex and isoelectric focusing. 2. All four endoglucanases hydrolysed CM-cellulose, H3PO4-swollen cellulose, cellotetraose and cellopentaose, but differed in the rate and mode of attack. 3. Attack on cotton fibre by the endoglucanases was minimal, but resulted in changes that were manifested by an increased capacity for the uptake of alkali, and a decrease in tensile strength. 4. All four endoglucanases acted synergistically with the exoglucanase [cellobiohydrolase; Wood & McCrae (1972) Biochem. J. 128, 1183-1192] of T. koningii during the early stages of the breakdown of cotton fibre, but only two could produce extensive solubilization of cotton cellulose when acting in admixture with the exoglucanase component. 5. The mode of action of the enzymes is discussed in relation to these synergistic effects. It is suggested that the results are compatible with the interpretation that the ‘crystalline’ areas of cotton cellulose are hydrolysed only by those endoglucanases capable of forming of forming an enzyme-enzyme complex with the cellobiohydrolase on the surface of the cellulose chains.

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