A peak near 420 nm interfering with the spectral detection of cytochrome P450 has been reported for invertebrates and fish. It has been variously suggested to be a breakdown product of P450, or a hemoprotein with unknown functions. Similar spectra were observed in the present work with a neotropical fish, an amphibian, and rodents. Comparative analysis showed that difference spectra resulted from an unknown hemoprotein and neither from P420, nor from hemoglobin, that may contaminate animal microsomes. Seasonal appearance of this protein was observed and its spectrum described. This protein completely substituted P450 in spectra of liver microsomes of fish and rodents collected in the summer, while in the winter the same animals displayed either the classic P450 spectra (rodents) or those accompanied with the low-intensity 421-nm peak (fish). We suggest that the compound visualized in P450 spectra is a functional protein and not an artifact. The possibility that an unknown protein may substitute for cytochrome P450 in microsomes under certain environmental conditions and play a role in animal adaptation to unfavorable environmental fluctuations is discussed.