According to the work of Slonimski, Acher, Péré, Sels & Somlo (1963), normal yeast contains two molecular species of cytochrome c, iso-1 and iso-2, the latter being the minor component; except for a few amino acid residues, the primary structures of these single-chain proteins are similar. These workers postulated that a precursor protein without haem of iso-2-cytochrome c exists in the cell before induction by oxygen, and that this protein is a specific repressor for the synthesis of the iso-1-cytochrome c polypeptide chain. In the present paper, three experiments are reported which support the possibility of a precursor protein of iso-2-cytochrome c. (1) At the beginning of induction, the rate of incorporation of radioactive amino acids into iso-2-cytochrome c (moles of a given radioactive amino acid incorporated per mole of corresponding amino acid residues in iso-cytochrome c) was lower than that of iso-1-cytochrome c by 30 to 40%, indicating that at least a part of iso-2-cytochrome c did not come from an exchangeable amino acid pool. (2) 5-Methyltryptophan inhibited the formation of both iso-1- and iso-2-cytochrome c; however, iso-1-cytochrome c synthesis was more sensitive to the presence of the analogue than is iso-2-cytochrome c synthesis; (3) in a phenylalanine- and tyrosine-requiring mutant, the formation of iso-2-cytochrome c was not suppressed in the absence of these amino acids, whereas the formation of iso-l-cytochrome c was severely affected. These data can be understood if a precursor protein for iso-2-cytochrome c exists in non-induced cells.