The bovine mitochondrial gene products ND2 and ND4, components of NADH dehydrogenase, have been purified from a chloroform/methanol extract of mitochondrial membranes, and the human mitochondrial gene products ND2 and cytochrome b have been obtained by similar procedures. They have been identified by comparison of their amino-terminal protein sequences with those predicted from DNA sequences of bovine and human mitochondrial DNA. All of the proteins have methionine as their amino-terminal residue. In bovine ND2, this residue is encoded by the "universal" isoleucine codon AUA, and the sequences of human cytochrome b and bovine ND2 demonstrate that AUA also encodes methionine in the elongation step of mitochondrial protein synthesis. In human ND2, the amino-terminal methionine is encoded by AUU, which, as in the "universal" genetic code, is also used as an isoleucine codon in elongation. Thus, AUU has a dual coding function which is dependent upon its context.