Six endoglucanases (Endo I; II; III; IV; V; VI), three exoglucanases (Exo I; II; III) and a β-glucosidase (β-gluc I) were isolated from a commercial cellulase preparation derived from Trichoderma viride, using gel filtration on Bio-Gel, anion exchange on DEAE-Bio-Gel A, cation exchange on SE-Sephadex and affinity chromatography on crystalline cellulose. Molecular masses were determined by polyacrylamide gel electrophoresis. One group of endoglucanases (Endo I, Endo II and Endo IV) with Mr of 50000, 45000 and 23500 were more random in their attack on carboxymethylcellulose than another group (Endo III, Endo V and Endo VI) showing Mr of 58000, 57000 and 53000 respectively. Endo III was identified as a new type of endoglucanase with relatively high activity on crystalline cellulose and moderate activity on carboxymethylcellulose. Exo II and Exo III with Mr of 60500 and 62000 respectively showed distinct adsorption affinities on a column of crystalline cellulose and could be eluted by a pH gradient to alkaline regions. These enzymes were cellobiohydrolases as judged by high-pressure liquid chromatography of the products obtained from incubation with H3PO4-swollen cellulose. It was concluded that these exoglucanases are primarily active on newly generated chain ends. Exo I was essentially another type of exoglucanase which in the first instance was able to split off a cellobiose molecule from a chain end and then hydrolyse this molecule in a second step to two glucose units. β-Gluc I was a new type of aryl-β-d-glucosidase which had no activity on cellobiose. The enzyme had a Mr of 76000 and was moderately active on CM-cellulose, crystalline cellulose and xylan and highly active on p-nitrophenyl-β-d-glucose and p-nitrophenyl-β-d-xylose.