1. The structures of mammalian cytochrome P-450 isoenzymes have been compared with respect to micro-sequence heterogeneity and their haem-binding cysteinyl peptides. 2. Mechanisms of induction of several P450 gene families are described including transcriptional activation and mRNA stabilization in cytochrome P450 I, II and IV families. 3. The tissue expression and substrate specificity of the cytochrome P450 IV family in liver, kidney and lung have been discussed. 4. The role of hepatic cytochrome P450 IVA1 induction in peroxisome proliferation is presented, and emphasis placed on the identification of susceptible and non-susceptible species.