In the present study, the diversity of a library of drug-metabolizing bacterial cytochrome P450 (P450) BM3 mutants was evaluated by a liquid chromatography-mass spectrometry (LC-MS)-based screening method. A strategy was designed to identify a minimal set of BM3 mutants that displays differences in regio- and stereoselectivities and is suitable to metabolize a large fraction of drug chemistry space. We first screened the activities of six structurally diverse BM3 mutants toward a library of 43 marketed drugs (encompassing a wide range of human P450 phenotypes, cLogP values, charges, and molecular weights) using a rapid LC-MS method with an automated method development and data-processing system. Significant differences in metabolic activity were found for the mutants tested and based on this drug library screen; nine structurally diverse probe drugs were selected that were subsequently used to study the metabolism of a library of 14 BM3 mutants in more detail. Using this alternative screening strategy, we were able to select a minimal set of BM3 mutants with high metabolic activities and diversity with respect to substrate specificity and regiospecificity that could produce both human relevant and BM3 unique drug metabolites. This panel of four mutants (M02, MT35, MT38, and MT43) was capable of producing P450-mediated metabolites for 41 of the 43 drugs tested while metabolizing 77% of the drugs by more than 20%. We observed this as the first step in our approach to use of bacterial P450 enzymes as general reagents for lead diversification in the drug development process and the biosynthesis of drug(-like) metabolites. Copyright © 2011 by The American Society for Pharmacology and Experimental Therapeutics.