Distinct and different molecular structural features are manifested by substrates, inhibitors and inducers of the two families of liver microsomal enzymes, the phenobarbital-induced cytochromes P-450 and the 3-methylcholanthrene-induced cytochromes P-448. In a theoretical study based on molecular orbital calculations and molecular graphics, it is established that cytochrome P-448 substrates contain fused aromatic or heteroaromatic rings giving rise to overall molecular planarity with relatively small molecular depth. In contrast, substrates of the cytochromes P-450 have greater conformational freedom and an ability to bind at more than one point of attachment, as a result of possession of certain characteristic functions, namely, a carbonyl and/or amine moiety coupled with an iso-propyl group, or similar function of equivalent shape and hydrophobicity. The implications are that the binding sites of cytochromes P-448 contain a number of hydrophobic aromatic amino acid residues orientated so as to allow occupation by similar substrates containing co-planar aromatic rings, whereas those of the phenobarbital-induced cytochromes P-450 contain hydrophilic amino acid residues capable of hydrogen bonding to greater than C = O moieties and at least one leucine or valine residue, as these contain the complementary isopropyl function. The corollary of these findings is the possibility of prediction of the toxicity of new chemicals on the basis of their molecular dimensions.