Oestrogen-like biological activity towards male fish has been demonstrated previously for degradation products of some detergents and industrial feed-stock chemicals, released into water environments. The endogenous oestrogen, 17 beta-oestradiol, is a particularly effective membrane-antioxidant, which may confer beneficial properties in addition to its genomic-mediated effects. Some of the most widely quoted environmental oestrogen-mimics were tested for liposomal membrane-antioxidant ability, measured as inhibition of lipid peroxidation. 4-Nonylphenol and bisphenor A (4,4-isopropylidenediphenol) were best able to mimic the antioxidant action of 17 beta-oestradiol by displaying effective inhibition of liposomal-membrane lipid peroxidation, whereas dibutylphthalate ester and phthalic acid diethylether (PADE) were considerably less effective antioxidants. The computer-based molecular modelling used in this study indicated a close structural similarity between these compounds and 17 beta-oestradiol (or the partial oestrogen/antioestrogen tamoxifen, a xenoestrogen drug used in breast cancer therapy), thus predicting their environmental action as oestrogen-mimics : genomic effects mediated by oestrogen receptor binding and non-genomic effects as membrane-antioxidants. Furthermore, 17 beta-oestradiol itself liberated initially into sewage-treatment lagoons from protein-bound 17 beta-oestradiol in human faeces, is now thought to be one of the main causes of the observed hermaphroditism of the male fish in the water-borne environment. Dietary phytoestrogens and their metabolites (also membrane-antioxidants) may have a similar activity in partial feminization, which could result from their excretion throughout the environment (aquatic and soil). Computer-assisted predictions have proved valuable in these comparative studies on these environmental xenoestrogens in the liposomal model-membrane system and could be generally utilized to monitor the likely extent of oestrogenic exposure after discharge of particular chemicals in industrial, domestic and sewage-treatment effluents. Paradoxically, a reduction in reproductive capacity in these fish may be, in part, compensated for by protection of their membranes against the oxidative stress generated by exposure to other pollutants in water environments.