The members of the CYP3A subfamily of cytochromes P450 play a key role in the detoxication of many therapeutic drugs and other xenobiotics and are also responsible for the bioactivation of a number of important human procarcinogens such as aflatoxin B1. A detailed structural model would prove invaluable in both understanding the catalytic mechanism and in predicting substrates and inhibitors of the enzyme. As yet no crystal structure for any mammalian P450 is available. In collaboration wih others we are developing a homology model for human CYP3A4 based on the existing structures of bacterial enzymes. We have the ability to refine this model experimentally as we are also examining the catalytic activity of variants of the enzyme produced by site-directed mutagenesis and in particular we are focusing upon interchanging residues between CYP3A4 and the homologous but less catalytically active, CYP3A5. To allow mechanistic interpretation of the results from such studies we are dependent upon the interactive graphics workstations and MidasPlus software of the Computer Graphics Laboratory which allows us to dock the substrates (such as aflatoxin B1) with the modeled enzyme and thus rationally predict catalytically important residues. At this stage we have obtained the coordinates of the latest homology model from Dr. Szklarz and, using MidasPlus for visualization, are selecting suitable residues for mutagenesis. In the near future we hope to use the Kuntz group's DOCK program to predict the key sites of interaction between aflatoxin B1 and the modeled protein.