We have expanded our studies of a patient with a mitochondrial myopathy caused by a defect at the level of complex III of the respiratory chain. Using activity measurements, electron microscopy, protein synthesis in the presence of emetine, and antibody binding, we have demonstrated that the defect is not expressed in cultured skin fibroblasts from this patient. Electron microscopy of peripheral blood leukocytes and activity measurements in transformed lymphoid cells indicated that the defect was not expressed in these cells either. These results imply that there are either isoforms of complex III components which show differential tissue expression or that independent segregation and assortment of defective mitochondria has occurred during development. © 1986.