Abstract: The blue-green algal blooms in Donghu Lake are formed by seven species: Microcystis aeruginosa, Microcystis aeruginosa var. major, Microcystis marginata, Anabaena circinalis, Anabaena flos-aquae, Oscillatoria formosa and Aphanizomenon sp. Through bioassay, we found that blooms of M. aeruginosa, M. aeruginosa var. major, M. marginata and A. flos-aquae are toxic, while those of A. circinalis and O. formosa are non-toxic. The toxicity and pigments of the water-bloom varied with seasons. From August to September, the Microcystis bloom showed the lowest toxicity, while the highest toxicity was detected in December(LD50 24.4mg/kg mouse wt.). Studies of the pharmacological characteristics of the toxin and the comparison of the clinical symptoms induced by these toxins through different routes(intracerebral and intraperitoneal)have also been made.