Cytochromes P-450 and P-448 in microsomal suspensions have been shown to be spectrally distinct by magnetic circular dichroism spectroscopy. Furthermore, this technique can be used to measure induction of these two cytochromes by phenobarbital and 3-methyl-cholanthrene. Magnetic circular dichroism spectroscopy is thus at least as useful as difference spectroscopy for the investigation of P-450 and P-448 and more informative because the presence of cytochrome b(5) and hemoglobin can be detected concurrently. We have also shown that the molar magnetic ellipticity for reduced + CO treated cytochrome P-450 of Pseudomonas grown on camphor is of similar value to that of reduced + CO treated microsomal P-450 and P-448.