The heme protein cytochrome P-450cam from Pseudomonas putida was investigated by Mössbauer spectroscopy in both oxidized and reduced states. The oxidized enzyme in the presence of the substrate, camphor, contains a mixture of high-spin (S = 5/2) and low-spin (S = ½) ferric heme sites. The high-spin fraction increases as temperature is raised. Removal of camphor results in a conversion from high spin to low spin. Hyperfine parameters that approximately describe the experimental spectra were calculated. Anaerobic reduction of P-450cam in camphor solution produces a high-spin ferrous (S = 2) state. Exposure of this preparation to oxygen results in a new complex whose Mössbauer spectra are similar to those observed for oxygenated hemoglobin. Both proteins show large quadrupole splitting and only moderate isomer shift relative to iron metal; no paramagnetic effects are observed even in large applied magnetic fields. Such spectra appear to be characteristic of the heme group with an O2 molecule as one axial ligand. P-450cam also forms a stable adduct with carbon monoxide. The Mössbauer spectra of this complex are very similar to those of hemoglobin carbon monoxide.