Harmful algal blooms have long been an issue worldwide owing to their adverse effects on drinking water treatment processes and drinking water quality. This study was conducted to investigate the removal of Microcystis aeruginosa in water by gamma irradiation. The effects of pH values and various additives (CH3OH, thiourea and NO3-) on M. aeruginosa removal and changes in antioxidant enzymes and lipid peroxidation were also investigated. The results revealed that 98% of the M. aeruginosa were removed on the 5th day after gamma irradiation at a dose of 9 kGy. Based on SEM observations, M. aeruginosa cells were injured by gamma irradiation. However, the presence of CH3OH, thiourea and NO3- attenuated the M. aeruginosa injury process. The removal of M. aeruginosa was found to be affected by solution pH, with higher removal efficiency occurring under acidic conditions than neutral or alkaline conditions. Carotenoids and phycocyanin photosynthetic pigment contents in M. aeruginosa cells were adversely affected by gamma irradiation, with their concentrations decreasing with increasing doses of gamma irradiation. Low gamma irradiation doses enhanced the activity of superoxide dismutase (SOD) and peroxidase (POD) in M. aeruginosa cells, while the activity of SOD and POD decreased with high doses. The lipid peroxidation in M. aeruginosa cells increased with increasing doses of irradiation. (C) 2011 Elsevier B.V. All rights reserved.