Circular dichroism (CD) spectra of ferric, ferrous and ferrous-carbonyl forms of Pseudomonas cytochrome c peroxidase have been recorded in the wave length range 200 to 650 nm. CD spectra in the Soret region show that in the oxidized enzyme the two hemes are degenerate, whereas in the reduced form the hemes are perturbed differently and one of the hemes appears to be non-degenerate. Changes in optical activity upon formation of the carbonylderivative suggest a spin-state conversion and indicate the presence of one high-spin and a low-spin heme. A histidine residue is proposed for the axial ligand of the heme iron. The alpha-helical content of the enzyme is estimated to be 34%. Ligand binding or changes in the oxidation state of the heme iron do not alter the conformation of the protein backbone. The dichroic spectra of oxidized and reduced cytochrome c-551 (P. aeruginosa) are included for comparison. In the visible region the cytochrome exhibits CD spectra similar to those of the peroxidase, whereas in the Soret region the dichroic spectra of the cytochrome are simpler. CD spectra in the far-ultraviolet region show the cytochrome to have a high alpha-helix content.