The phenobarbital-inducible form of cytochrome P-450 purified from rabbit liver microsomes is phosphorylated by cAMP-dependent protein kinase at a single site, the serine residue in position 128 of the amino acid sequence. The serine is located in a characteristic recognition sequence for cAMP-dependent protein kinase and is part of a primary structure which is conserved during evolution, present also in phenobarbital-inducible rat cytochrome and cytochrome P-450 CAM from Pseudomonas putida. The contribution of these findings to our understanding of the structure and membrane topology of cytochrome P-450 LM2 and its turnover regulated by phosphorylation is discussed.