The determination of fiber types is routinely accomplished in skeletal muscle biopsy specimens by enzymatic histochemical analysis, which detects adenosine triphosphatase (ATPase) activity on cryostat sections. This study assesses postmortem antigen degradation, the effects of fixation and processing, and the neuropathologic applications of MY-32, a monoclonal antibody to fast twitch skeletal myosin. Formalin-fixed, paraffin-embedded sections of skeletal muscle biopsy specimens obtained from the quadriceps femoris were immunoreacted with this antibody. Cryostat sections of the same muscle biopsy specimens were examined after brief fixation in either acetone or formalin. Parallel cryostat sections of frozen muscle were also assessed with ATPase preparations at pH 9.4 and 4.3. To evaluate the effect of postmortem interval and autolysis on antigen degradation, skeletal muscle samples obtained at 12 hours postmortem were immunoreacted after 12, 24, or 36 additional hours. These specimens were examined as immunoreacted cryostat sections and compared with parallel sections reacted for ATPase at pH 9.4 and 4.3. Representative sections from each time point were also fixed in formalin, routinely processed, paraffin embedded, and immunoreacted. Selected muscle biopsy specimens with a range of neuropathologic diagnoses, including fiber type grouping, Type II atrophy, and congenital fiber type disproportion, were also assessed for immunoreactivity. Our results indicate that the MY-32 monoclonal antibody specifically reacts with Type II (fast twitch) fibers. Immunoreactivity is most intense in cryostat sections immersion fixed in acetone, but moderately intense, specific immunoreactivity can be clearly identified in formalin-fixed (frozen or paraffin-embedded) tissue obtained even 48 hours after death. Application of this nonenzymatic method for fiber type determinations in the neuropathologic evaluation of skeletal muscle biopsies is presented.