Whether the same or distinct steroid sulphatases (STS) are involved in the hydrolysis of alkyl and aryl steroid sulphates remains controversial. We have examined the ability of a placental steroid sulphatase to hydrolyse oestrone sulphate and/or dehydroepiandrosterone sulphate (DHA-S) by expressing the enzyme in COS-1 cells. Using either intact cells or broken cell preparations, the expressed sulphatase was found to hydrolyse both oestrone sulphate and DHA-S. The catalysis of oestrone sulphate and DHA-S by the expressed sulphatase was almost completely abolished by the steroid sulphatase inhibitor, oestrone-3-O-sulphamate. It is concluded that both alkyl and aryl steroid sulphates can be hydrolysed by the same steroid sulphatase.