THE DNA of mammalian mitochondria (mtDNA) exists as a single class of circular molecules about 5 µm long1,2, and codes for two ribosomal RNA species and several transfer (4S) RNA components3,4. The function of the remaining 80% of the genome is unknown, although it has been suggested that the residual coding capacity provides messenger RNA (mRNA) which is translated exclusively by mitochondrial ribosomes producing about ten protein components which are associated with the inner mitochondrial membrane. Not all protein synthesis in mitochondria, however, may be directed by mtDNA—some could represent translation products of nuclear-coded messages imported into the organelles5,6. We report here on differences between electrophoretic profiles of proteins synthesised in mitochondria of a wider variety of animal cell lines. Evidence is also presented suggesting that mtDNA has a role in determining the pattern of proteins observed.