We examined the substantia nigra of 8 patients with Parkinson's disease immunohistochemically using antisera against complexes I, II, III, and IV of the mitochondrial electron transport system. In the patients with Parkinson's disease, a fair proportion of the nigral neurons showed reduced staining against the complex I antibody. The proportion of the neurons with reduced staining ranged from 12.7 to 74.1% of the melanized nigral neurons. Although neurons with reduced immunostaining for complex I were also observed in control subjects, the proportion among the nigral neurons was significantly smaller than in parkinsonian patients. Staining for complexes III and IV appeared normal. Staining of substantia nigra for complex II was decreased in 3 parkinsonian patients. These results are consistent with our findings that there is a deletion of gene coding for the four subunits in the mitochondrial DNA located in the striata of parkinsonian patients.