The circular-dichroism spectra of baker's-yeast iso-1- (methylated and unmethylated forms) and iso-2-cytochrome c species were examined between 200 and 600nm. In the visible region the yeast haemoproteins have characteristics nearly indistinguishable from those of horse heart cytochrome c. From the spectra in the u.v. region the latter appears, however, to be more helical. It is proposed that the likely element of non-helical structure in iso-1-cytochrome c is residues 62-70.