Erythromycin, a macrolide antibiotic, is commonly used in human life. This compound and its derivatives have been detected in various aquatic compartments and may pose a serious threat to aquatic organisms. This study investigated the effects of erythromycin on the growth, antioxidant system and photosynthesis of Microcystis flos-aquae. The results showed that at 0.001-0.1 mu g L-1, erythromycin could stimulate the growth of M. flos-aquae and increase its photosynthetic activity; however, it did not significantly increase the activities of superoxide dismutase (SOD) and catalase (CAT) or the levels of malondialdehyde (MDA) and reactive oxygen species (ROS). In contrast, the growth of M. flos-aquae was significantly inhibited (p < 0.01) at high levels of erythromycin, reaching an inhibition rate of 81.6% at 40 mu g L-1 erythromycin. At the same time, the activities of SOD and CAT along with MDA content also increased significantly (p < 0.01), indicating that the high concentrations of erythromycin caused a severe oxidative stress on algae. However, the balance between oxidants and antioxidant enzymes were disrupted because ROS content simultaneously increased. In addition, the fluorescence parameters of M. flos-aquae decreased significantly with both exposure time and increasing concentration of erythromycin, indicating that photosynthesis was inhibited. (C) 2014 Elsevier B.V. All rights reserved.