The phytoestrogenic activity of alfalfa irrigated with sewage water was determined by radioreceptor assay (RRA) and high pressure liquid chromatography (HPLC). It was found that irrigation of alfalfa fields increased the phytoestrogenic activity measured in the leaves from 10 ng estrogen equivalents (est. eqv.) to 137 ng est. eqv. g-1, which was principally due to a rise in coumestrol (<10 μg to 60 μg g-1). Similarly, when plants grown in greenhouses under controlled conditions were irrigated with sewage water, the phytoestrogenic content rose from 34 to 48 est. eqv. g-1; (P<0.01; n = 16). Under hydroponic conditions, addition of 1 nM of estrone to the media resulted in a significant rise in phytoestrogen content in the leaves within 48 hr (from 21.2 to 34.5 est. eqv. g-1; P<0.05, n = 10). Alfalfa irrigated with sewage water may therefore have levels of coumestrol (<30 μg g-1) which can affect fertility in cattle. The agents in the sewage water responsible for increased phytoestrogen content of the alfalfa are probably the steroidal estrogens, estradiol and estrone, which are present in sewage in concentrations ranging from 10 to 300 ng l-1.