1. The cytochromes P-450 are of interest in fields as diverse as biochemistry, pharmacology, mechanistic and synthetic chemistry, carcinogenesis, toxicology, endocrinology, and nutrition. 2. Cytochrome P-450 enzymes are coded by a multi-gene family. 3. The many forms have some elements of similarity in their primary sequences and catalyze chemically equivalent reactions, either mixed-function oxidations or reductions. 4. The catalytic specificity of the individual forms of the enzyme is related to three-dimensional interactions between individual substrates and their binding sites, and the reactions are related in their chemistry. 5. Regulation of enzyme levels appears to involve transcriptional and post-transcriptional aspects.