Using the postulated mechanism for the enzyme estrone sulfatase (ES), we have determined a possible transition state for the reaction catalysed by ES as a representation of the active site. Using the derived structure, we have undertaken the molecular modelling of several steroidal and non-steroidal inhibitors in an attempt to rationalise the inhibitory activity of a number of potent inhibitors. (C) 2001 Elsevier Science Ltd. All rights reserved.