Abstract Upon addition of oxygen to the ferrous form of bacterial cytochrome P-450 (P-450 cam), a new spectral species of the enzyme was observed having absorption maxima at 555, 418 and 355 nm. This new species of P-450 cam could be converted into carbon monoxy-P-450 cam by the displacement of oxygen with carbon monoxide. Conversely, the carbon monoxide derivative could be transformed into the new species by extensive bubbling of the reaction mixture with oxygen. The EPR signal at g = 2.26, characteristic of low spin ferric P-450 cam, is absent in the new spectral species. Based on these findings, the new spectral species was interpreted to be an oxygenated form of P-450 cam, probably an intermediate in the overall hydroxylation reaction.