Chronic oral toxicity of a low dose of microcystin‐LR (MCLR) was examined in female BALB/c mice for 18 months. Six‐week‐old female mice received 20 μg/L of the toxin in drinking water, which is about 200‐fold higher than the level in contaminated drinking water. Control mice received water alone. Mortality, clinical signs, body weights, and food and water consumptions were recorded during the study. Examinations on hematology, serum biochemistry, necropsy, organ weights, and histopathology were performed at months, 3, 6, 12, and 18. The immunohistochemical distribution of MCLR was examined in the liver at these time points. Mean cumulative MCLR intake after 18 months was estimated at 35.5 μg per mouse. The present test indicates that administration of a low dose of MCLR in drinking water resulted in neither chronic toxicity nor accumulation of the toxin in the liver. Based on previous epidemiological studies and the present chronic toxicity test, we recommend 0.01 μg/L as an maximum acceptable level for microcystins in drinking water, applying a safety factor of more than 1000. ©1999 John Wiley & Sons, Inc. Environ Toxicol 14: 45–55, 1999