A unialgal culture of Microcystis aeruginosa Kütz. emend. Elenkin, which is toxic when injected intraperitoneally into white mice, has been isolated. It produces a complex of symptoms similar to those which have been described for a number of toxic waterblooms. Two distinct factors seem to be present. One causes death in 4 to 48 hours preceded by symptoms of piloerection and dyspnea. It is detected only with freshly harvested cells. The other causes death within 2 to 3 hours preceded by pallor and convulsions. This second factor is an endotoxin which is detected only when cells become leaky or disintegrate. The presence of the slow-death factor tends to be obscured by the release of the fast-death factor. The concentration of the fast-death factor in the cells varies with the developmental stage of a culture and at maximum can be just as high as that of a very toxic waterbloom.