Abstract: In recent years, due to the eutrophication of water-bodies, some lakes and resevoirs that serve as the sources for water supply have been heavily contaminated by water bloom. It was demonstrated abroad that some species of the blue-green algae produce toxic substances having harmful effects on domestic animals. Those blue-green algae reported to produce toxic substances are also found in lakes in our country. Therefore, it is important to clarify whether they are really toxic or not.So far as we know, this study is the first one using hemagglutination test and mouse assay method to detect the toxicity of blue-green algae in China. The samples of water bloom collected from the Donghu Lake from late August to mid October in 1984 were examined. Six out of fourteen samples showed toxicity to experimental animals. The dominant species of the bloom was Microcystis aeruginosa.