The phthalate esters represent a class of chemicals used widely and diversely in industry. Concern that phthalates might be oestrogenic arose from observations that the diesters inhibited the binding of 17beta-estradiol to isolated oestrogen receptors and stimulated the expression of cellular oestrogen-sensitive endpoints (gene expression, mitosis) in vitro. However, conflicting results have been found in comparable studies, and those studies that have demonstrated oestrogen mimicry have generally done so at concentrations approaching, or above, the limit of water solubility for the phthalates. The monoesters (the primary metabolites of the diesters in vivo) are inactive in similar in vitro tests. Furthermore, the diesters have not shown any oestrogenic activity in numerous and diverse studies in vivo at doses eliciting systemic toxicity. Consequently, the oestrogenic activity of phthalates identified in in vitro studies is not relevant to humans or the environment.