In Krebs ascites-tumour cells, cytochrome c is segregated in the mitochondria and the level in microsomes could not be measured. At 22° in glucose–buffer Krebs cells synthesized a spectrum of proteins including cytochrome c. Mild osmotic shock in the presence of ribonuclease had little effect on incorporation of [14C]-leucine or [14C]valine into mixed mitochondrial protein but strongly inhibited synthesis of non-mitochondrial cytoplasmic proteins. Under these conditions, labelling of cytochrome c was also strongly inhibited. After pulse labelling of Krebs cells at 22° for 10min. the cytcchrome radioactivity found in mitochondria was higher than in microsomes. After addition of unlabelled amino acid as ‘chase’ there was 137% increase in radioactivity of cytochrome c but only a 3% increase in radioactivity of whole-cell protein. It is concluded that the peptide chain of cytochome c is synthesized on cytoplasmic ribosomes. Mitochondria therefore do not have the character of self-replicating entities, but are formed by the cooperative function of messenger RNA of cytoplasmic ribosomes and, possibly, of intramitochondrial messenger derived from the mitochondrial DNA.