This investigation deals with the immunocytochemical localization of Cu/Zn superoxide dismutase (SOD) in the spinal cord neurons of transgenic mice that overexpress Gly93Ala mutant human Cu/Zn SOD and demonstrate clinicopathological features similar to human amyotrophic lateral sclerosis (ALS) with Cu/Zn SOD mutation. At low magnification of light microscopy, the gray and white matter of the spinal cord of Gly93Ala mice showed more intense Cu/Zn SOD immunoreactivity than that of control mice. At higher magnification, the cytoplasm of control mice neurons displayed a distinct staining for Cu/Zn SOD, whereas the surrounding neuropil was only weakly stained. In contrast, the intensity of Cu/Zn SOD immunoreactivity in the cytoplasm of the majority of Gly93Ala mice neurons was similar to that in the neuropil. Almost all neuronal hyaline inclusions (NHIs) of Gly93Ala mice were positively immunostained by antibodies to Cu/Zn SOD, ubiquitin and phosphorylated neurofilament protein (NFP), the intensities of which were much higher in the NHIs than in the surrounding cytoplasm. In control mice, significant Cu/Zn SOD precipitation was not observed to be limited to any particular region of the neuronal cytoplasm. Intracytoplasmic vacuoles in the neuronal soma and processes of Gly93Ala mice were not stained by any of these antibodies. These results indicate that Cu/Zn SOD colocalizes with ubiquitin and phosphorylated NFP in NHIs of mice expressing mutant Cu/Zn SOD; similar findings have been shown for Lewy body-like inclusions of familial ALS patients with Cu/Zn SOD mutation. Moreover, our results point to the possibility that Cu/Zn SOD mutation may have a role in the abnormal Cu/Zn SOD accumulation in the NHIs, in association with motor neuron degeneration.