A monoclonal antibody (MAbS1) specific for exo-beta-1,4-glucanase A (ExoA) from Ruminococcus flavefaciens FD-1 was generated from homogeneous enzyme. Immunoblots showed that MAbS1 reacted strongly with the ExoA polypeptide and weakly with other polypeptides in the crude cellulase preparation from this organism. However, immunoblots of V8 protease-digested crude cellulase preparations suggest that MAbS1 also recognizes proteins in R. flavefaciens FD-1 other than ExoA, or that ExoA exists in multiple forms. Immunoprecipitations using MAbS1 and crude cellulase preparations specifically removed p-nitrophenyl-beta-D-cellobiosidase (PNPCase) activity, with little removal of carboxymethylcellulase or C-14-cellulase activities. Immunoprecipitation pellets contained the ExoA polypeptide as well as two lower-molecular-weight polypeptides which were present in much lower concentrations. The antibody is also suitably specific towards ExoA for use in affinity purification ofthe exoglucanase from culture supernatants of R. flavefaciens FD-1. MabS1 inhibits PNPCase (exoglucanase) activity in vitro. Moreover, the antibody when added to growing cultures of R. flavefaciens FD-1 decreased the rate and extent of cellulose hydrolysis. These data suggest that exoglucanase A is an important component ofthe R. flavefaciens FD-1 cellulase system. The results show that a specific monoclonal antibody directed rewards ExoA inhibits the catalytic activity of this enzyme and decreases the rate and extent of cellulolysis by this microorganism. Thus, ExoA is a vital component of the cellulase system of R. flavefaciens FD-1, and its activity may be a rate-limiting step in cellulolysis.