icrocystins produced by cyanobacterial blooms have been extensively found in water reservoirs used for drinking water abstraction and many conventional water treatment technologies have been reported to be ineffective for removing them. Safe barriers against microcystins in drinking water, are therefore, needed. This paper investigates the removal of the most commonly found microcystin variants by a negatively charged nanofiltration (NF) membrane. Different electrolyte solutions (KCl and CaCl2), pH (5-8.5) and types of natural organic matter (NOM) were used in order to study the influence of chemical feed characteristics on NF performance for mycrocistins removal. Three types of water samples were employed, ozonated water and decanted water (DW) from a water treatment plant, and DW spiked with Aldrich humic acid and salicylic acid, due to the lack of hydrophobic, high molecular weight NOM in the selected natural waters. Microcystins revealed a strong membrane fouling ability for 150 mu g/L as total microcystins, being this concentration one order of magnitude higher than those of natural occurring blooms. For 16 mu Lg/L as total microcystins, the fouling behaviour was attenuated. All microcystin variants studied (MC-LR, MC-LY and MC-LF) were almost completely removed by NF (>97%). The NOM type and concentration, as well as the background inorganic matrix (pH and 1 mM CaCl2) showed no influence on the NF performance to remove microcystins. Microcystins concentrations in the NF permeate were always far below the drinking water guideline value of 1 mu g/L MC-LR adopted by World Health Organisation, and usually below the quantification limit. Excellent water quality was also achieved in terms of turbidity (<0.13 NTU) and NOM content (DOC <= 1 mg C/L, UV254 nm < 0.002 cm(-1)). (C) 2005 Elsevier B.V. All rights reserved.