Abstract: Objective To investigate the level of microcystins(MC)in source and drinking water of a city along Lake Tiahu where toxic water blooms frequently occurred.Methods An enzyme-linked immunosorbent assay(ELISA)with a detection limit of 50ng/L,was used to monitor MC in water samples collected in J uly 2001 from 10 regions of Wuxi city,Jiangsu province in China.Of the 90 samples obtained,each 11 samples were raw and produced water from 7 waterworks that using lake or deep-well water as water sources,48 were household tap water,17 were shallow-well water,and 3 were river water.Results Heavy contamination with MC was observed in all 8 raw water samples from lake Taihu,with a range of 2 699 -14 188ng/L.Trace amount of MC(less than 100ng/L)remained in the treated and household tap water that produced from the lake water.No MC was detected in raw and treated deep-well water.MC was also detected in 5 of 17 shallow-well water,with a range of 64-323ng/L.Only 1 river water was positive for MC.Conclusion Regular water treatments might not be sufficient to remove MC from drinking water during bloom situations.More detailed investigations were required to evaluate the risk of MC contamination in the water supplied in this area.