A continuous hybrid cell line was derived that secretes monoclonal antibody capable of inhibiting the biological activity of mouse beta-nerve growth factor (beta-NGF). Results obtained with monovalent fragments indicate that the monoclonal antibody inhibits activity by interfering with the direct interaction between beta-NGF and the cell membrane receptor rather than by precipitating the dimeric form of beta-NGF. This monoclonal antibody binds to an antigenic determinant common to mouse beta-NGF, snake venom (Naja naja) beta-NGF, and human beta-NGF. These antibodies should provide specific molecular probes for a variety of studies of nerve growth factor including its tissue distribution and mechanism of action.