Types 1 and 2C fibers in human skeletal muscle were cross-reactively identified with monoclonal anti-bovine neurofilament (200 kd) antibody. Thirty seven biopsy samples including sixteen vastus lateralis muscles, twelve lumbar paravertebral muscles, six gluteus medius muscles, two flexor carpi ulnaris muscles, and one flexor pollicis longus muscle, were examined. Serial transverse sections were stained histochemically with myofibrillar ATPase (pH 10.4, 4.6, 4.3) and DPNH-tetrazolium reductase reactions, and immunochemically using the avidin-biotin-peroxidase complex with the primary antibodies of monoclonal anti-bovine neurofilament (200 kd, 160 kd, 70 kd) antibodies and anti-bovine glial filament acidic protein antibody. The immunochemical reaction with anti-NF (200 kd) antibody could distinguish two kinds of fibers; positive and negative in all of the specimens. No fiber was recognized with other antibodies. Myosin ATPase reactions in serial sections proved that the positively stained fibers with anti-NF (200 kd) antibody were types 1 and 2C fibers and negative fibers types 2A and 2B fibers. At present, it is not known what substance is responsible for the cross-reaction with the monoclonal anti-NF (200 kd) antibody in types 1 and 2C fibers, but this unique antibody would be valuable in two aspects: one concerns the problem of the evolution of fiber types, and the other the utility as another supplemental method to conventional myosin ATPase scheme.