Characteristics of the cellulolytic system of the anaerobic fungus Piromyces sp. strain E2 with respect to adsorption onto microcrystalline cellulose were examined. Cellulolytic enzymes were separated by gel filtration chromatography into a high-molecular-mass complex with an apparent mass of approximately 1,200 to 1,400 kDa and proteins of lower molecular weights. Adsorption of cellulolytic enzymes was not only very fast (within 2 min, equilibrium was attained) but also very effective: Avicelase, endoglucanase, and beta-glucosidase activities from the high-molecular-mass complex were almost completely removed by Avicel. Adsorption of these enzyme activities was proportional and appeared to obey the Langmuir isotherm. For Avicelase, endoglucanase, and beta-glucosidase activities, the maximum amounts adsorbed (Amax) and apparent adsorption constants (Kad) were 16.8, 600, and 33.5 IU/g and 284, 6.93 and 126 ml/IU, respectively. The results of this study strongly support the existence of a multiprotein enzyme complex. This complex was found not to be specifically associated with cell wall fragments as judged by chitin determination.