The changes in axons seen following exposure to 2,5-hexanedione and its metabolites have to date been studied using electron microscopy and conventional silver impregnation techniques. These have their draw-backs and do not identify the molecular components of a pathological change. We have used monoclonal antibodies to neurofilament proteins and tubulin with immunohistochemical techniques. Sprague-Dawley rats were given 2,5-hexanediol in their drinking water. After 2 weeks the rats showed signs of central and peripheral neuropathy, firstly in the hind limbs and this progressed with time of exposure. After 6 weeks, the administration of 2,5-hexanediol was ceased. Many of the rats were by this time paralyzed in the hind limbs. One third of the rats were then sacrificed. The remaining rats were allowed to recover for a 5 or 10 week period. The axonal changes were studied using an indirect immunoperoxidase method with monoclonal antibodies to the neurofilament proteins and tubulin. The injuries were simply, distinctly and reproducibly demonstrated. With this method it was possible to state that the excess material in the axon swellings was formed of neurofilament proteins and not tubulin. New findings such as the grouping of axon swellings in specific areas of cerebral cortex are reported. This report shows the advantage of using immunohistochemical techniques with monoclonal antibodies. These are easily reproducible and dependable methods for demonstrating pathological changes in the neuronal cytoskeleton.