1. A novel modelling alignment for P450s, utilizing NADPH-P450 reductase for electron transfer, is proposed on the basis of analysis of their amino acid sequences. 2. Information used to facilitate the alignment process includes: the recent X-ray crystal structure of P450102 (P450bm3), site-directed mutagenesis experiments, chemical modification of specific residues, and antibody recognition studies. 3. The alignment has been used to construct a number of microsomal P450s of relevance to xenobiotic and endogenous metabolism.