This International Standard specifies a method for the determination and quantification of microcystins in raw water (containing biomass) and treated water, such as tap water. The method described is validated for MCYST-RR, MCYST-YR, and MCYST-LR. It is also applicable for the determination of several structure variants[1] of these microcystins, but an unambiguous identification cannot be made due to the lack of commercially available standards and due to co-elution. The threshold value of 1 μg/l of MCYST-LR in water, proposed by the World Health Organization, can be followed after microcystin enrichment using solid phase extraction (SPE).