Immunohistochemical studies were performed in 18 hyperfunctional parathyroids with oxyphil cell aggregates for the detection of cytochrome-c-oxidase (complex IV of the respiratory chain), mitochondrial DNA polymerase gamma and human mitochondrial transcription factor A (h-mtTFA). Seventy-three oxyphil areas exhibiting a defect of cytochrome-c-oxidase were found. The defect involved both the mitochondrially coded subunits II/III and the nuclear derived subunits Vab. There was no loss of mtDNA polymerase gamma or of h-mtTFA in these foci, corresponding to a high content of mtDNA revealed by in situ hybridization. Isolated defects of h-mtTFA were also not found. In contrast, isolated defects of mtDNA polymerase gamma were present in 22 oxyphil foci. These results show that defects of cytochrome-c-oxidase in oxyphil cells are not due to altered expression of h-mtTFA or DNA polymerase gamma, indicating that other nuclear factors involved in the generation of the respiratory chain may be impaired. The low incidence of defects of mtDNA polymerase gamma and the absence of alterations of h-mtTFA and cytochrome-c-oxidase in these foci suggest that defects of mtDNA polymerase gamma are of minor pathogenetic significance.