Carbon monoxide difference spectra and pyridine binding spectra indicate the presence of cytochrome P-450/420 in plasma membranes from cauliflower inflorescences. Mild lithium dodecylsulfate polyacrylamide gel electrophoresis shows only one heme staining band in the plasma membrane fraction at an apparent molecular weight of 93 kiloDalton. This band is suggested to be due to a cytochrome P-450/420 dimer, in view of the known molecular weights of animal cytochromes P-450/420. The plasma membrane-bound cytochrome P-450/420 is probably identical to the blue-light-reducible b-type cytochrome of plant plasma membranes, which has been inferred to have a role in photomorphogenesis.