1. The cellulolytic enzyme (p-1 ,4-glucan cellobiohydrolase) isolated from culture filtrates of the fungus Trichoderma viride as described previously by us was further characterized as regards chemical, physico-chemical and enzymatic properties.2. The amino-acid composition revealed a high content of acidic amino acids (21.0%) in contrast to a low content of basic amino acids (5.5 %). The half-cystine content was 16 residues per molecule indicating the presence of 8 disulfide bridges, since no free sulfhydryl group was detectable. Alanine was found to be the N-terminal amino acid. 3. The enzyme contained 9.2 %, carbohydrate, which is most probably covalently bound to the protein. The constituent carbohydrates were identified as mannose, glucose and galactose.4. A molecular weight of 41 800 was determined by sedimentation equilibrium analysis. The data from the amino-acid and the carbohydrate analyses indicated a molecular weight of about 42 000.5. The pH-optimum for the enzyme was about 4.8 and thermostability studies showed that the enzyme was completely denatured after 3-min incubation at 78 "C.6. Experimental results demonstrated the enzyme to be a p-1,4-glucan cellobiohydrolase.7. The enzyme was capable of about 80 % degradation of microcrystalline cellulose within 72 h, if the reaction products (mainly cellobiose) were continuously removed by ultrafiltration. The rate of degradation of microcrystalline cellulose by the enzyme could be enhanced by addition of P-glucosidase. The activity of the enzyme was also increased by addition of endo-glucanase.