The interaction between the human aromatase enzyme and some organotins was investigated. Tributyltin (TBT) at 12 and 59 microM and dibutyltin at 74 microM inhibited aromatase activity in vitro but monobutyltin and tri-, di- and monooctyltins were without effect. In four separate kinetic studies of aromatase, the K(m(app)) for testosterone was 0.24, 0.21, 0.16 and 0.24 microM. TBT inhibited aromatase activity by causing the K(m(app)) to be increased without affecting the V(max), indicative of competitive inhibition. Slope and intercept replots confirmed the effect of aromatase on the K(m(app)). Slope replots from three separate kinetic studies provided Ki values for TBT of 64.5, 40.9 and 37.3 microM. Consequently, TBT is a competitive inhibitor of human aromatase with a Ki approximately 300-fold the K(m(app)) value.