Cellulose hydrolysis was found to be inhibited by a purified preparation of a waterextractable inhibitor from fresh sericea forage. This inhibition was obtained under rumen conditions as well as in in vitro trials. When casein was added to the cellulose-rumen fluid mixture containing PSI, considerable protection of activity was afforded to the cellulose degrading system; but at high levels of inhibitor, casein had no effect. While fine chopping of the fresh material or drying of the forage reduced the amount of extractable inhibitor to a low level, the digestibility of the cellulose of the dry form was not higher than the fresh forage.