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A selective inhibitor of aromatase (estrogen synthetase) would be a useful pharmacological tool with potential therapeutic application. We have found that 6 alpha-bromoandrostenedione (6 alpha-BrA) is a competitive inhibitor of human placental aromatase with respect to androstenedione, with an apparent Ki of 3.4 nM, while 6 beta-BrA is a mechanism-based irreversible inhibitor with an apparent Ki of 0.8 microM and a kinact of 0.025 min-1. Aromatase activity was measured by tritium release into water from the 1 beta position of [1(-3)H,4(-14)C]androstenedione in reaction mixtures containing NADPH and the aromatase.