The biochemical, histological and ultrastructural effects of 2,4-dinitrophenol and the calcium ionophore, A23187, on rat soleus muscle incubated in vitro have been examined to test the hypothesis that immunohistochemical techniques can be used to recognize early structural features of fibre damage. In control muscles, despite mild glycogen depletion and a mild reduction in protein synthetic rate in the central portion of the muscle, fibres throughout the muscle appear to be viable with normal cytoskeletal and contractile protein architecture, normal concentrations of high energy phosphates and no creatine kinase efflux. Dinitrophenol causes rapid creatine kinase efflux, extensive loss of immunolabelling for desmin and dystrophin, and abnormal myosin immunolabelling. Creatine kinase efflux and the changes in desmin and dystrophin are reduced by the exclusion of calcium. A23187 causes more gradual creatine kinase efflux associated with changes in myosin immunolabelling, but loss of desmin and dystrophin immunolabelling is restricted to a few of the most peripheral fibres. The results suggest that immunohistochemical methods can be used to reveal differences in the intracellular mechanisms of muscle damage. Although both dinitrophenol and A23187 may act, in part, through calcium-mediated processes, their effects on cytoskeletal proteins differ. Creatine kinase efflux after A23187 may not be due to gross sarcolemmal damage.