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Twentyfive cyanobacterial blooms in Lake Ladoga and adjacent water bodies were studied in the summer of 1990-1992 . Toxicity of the water bloom material for mice was detected in 9 cases . The maximal tolerable doses (MTD) of the material extracted from biomass varied within 3-30 mg kg -1 mouse body weight; 50% lethal doses (LD5o) were within 45-125 mg kg -1 . Toxic water blooms were registered in Karelian lakes and in the Neva Bay, Gulf of Finland . Cyanobacterial samples collected on the eastern coast of Lake Ladoga proved to be non-toxic . The species identified in toxic bloom material included Anabaena circinalis, A . flos-aquae, A . lemmermannii, Anabaena sp ., Aphanizomenon flos-aquae, Gloeotrichia echinulata, G. pisum, Microcystis aeruginosa and Oscillatoria sp . These data suggest that toxic forms of cyanobacteria are widespread in Karelian lakes belonging to the drainage basin of Lake Ladoga .

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