A prevalent explanation of the enzymic hydrolysis of cellulose involves at least two enzymes, cellulase and cellobiase. Cellulase produces and probably other oligosaccharides, whereas cellobiase hydrolyzes these products to glucose. In an effort to characterize the enzyme concerned in the hydrolysis of cellulose substitutes, the experiments were carried out by the following method. Seven milliliters of citrate (pH 4.5) containing 30 mg of the cellulose was incubated with 30 mg of Rhizopus mold cellulase (Sigma) and 2 drops of toluene at $47^{circ}$ continuing the aeration. After 24 hours the suspension was filtrated and a 1 ml aliquot of the filtrate was assyed. The activity is expressed in milligrams of reducing sugar as glucose. In general, native cellulose exhibits a much greater resistance to enzymic hydrolysis than do regenerated cellulose, Hence, it has become customary to use treated celluloses for measurement of cellulolytic activity, since the sensitivity is thereby greately increased. Accordingly, the author used cellulose substitutes such as CMC, cellulose phosphate, DEAE cellulose and alpha-cellulose (fiber) and Sigm acell (macrocrystalline) as substrate to investigate the effects of metal ions and thiol containing compounds on Rhizopus mold cellulase. The preferability of buffer system, temperature stability, and pH stability were studied, too. The following results were obtained; The cellulase is stable over the pH range of 2.8-6.0 and the temperature range of $70^{circ}-90^{circ}$. The cellulase was shown to be inhibited by sodium phosphate and $Cu^{2+}$, but not by $Fe^{3+}$ and EDTA. It was found that the addition of bovine albumin, $Mg^{2+}$, $Ca^{2+}$ and cysteine activated the cellulase activity.