1. The C1 component that was isolated from a Trichoderma koningii cellulase preparation (Wood, 1968) by chromatography on DEAE-Sephadex with a salt gradient was still associated with a trace of CM-cellulase activity (determined by reducing-sugar and viscometric methods). 2. Further chromatography on DEAE-Sephadex, with a pH gradient instead of a salt gradient, provided a C1 component that could still produce reducing sugars from a solution of CM-cellulose (to a very limited extent), but which could no longer decrease the viscosity (i.e. under the assay conditions employed). 3. No evidence for the non-identity of C1 component and the trace of CM-cellulase activity could be found when electrofocusing was done in a stabilized pH gradient covering three pH units (pH3–6) or, alternatively, only 0.5 pH unit (pH3.72–4.25). 4. The two protein peaks that were separated by electrofocusing in carrier ampholytes covering only 0.5 pH unit (isoelectric pH values of 3.80 and 3.95) were shown to be isoenzymes of the C1 component: they differed in the extent to which they were associated with carbohydrate (9% and 33%). 5. The purified C1 component had little ability to attack CM-cellulose or highly ordered forms of cellulose, but degraded phosphoric acid-swollen cellulose readily: cellobiose was the principal product of the hydrolysis (97%). 6. Dewaxed cotton fibre was degraded to the extent of 15% when exposed to high concentrations of C1 component over a prolonged period: cellobiose was again the principal sugar present in the supernatant (96%). 7. Cellotetraose and cellohexaose were hydrolysed almost exclusively to cellobiose. 8. Evidence indicates that the C1 component is a β-1,4-glucan cellobiosylhydrolase.
