Unialgal cultures of several species and strains of blue-green algae, including those most suspected of causing animal deaths, have been grown and found to vary greatly in toxicity. At least four toxic factors have been recognized. One produces fast deaths and is algal in origin. The others produce slow deaths and are bacterial in origin. The fast-death factor (FDF) is an endotoxin that so far has been encountered only with strains of Microcystis aeruginosa Kütz. emend. Elenkin. Its production is genetically and physiologically controlled. An FDF-producing strain of M. aeruginosa has been cultured on a large scale and the cells shown to be toxic when administered orally to sheep, calves, and smaller animals. FDF isolated from these cells has been identified as a quite stable cyclic polypeptide having an intraperitoneal LD(50) for white mice of 0.47 mg. per kg. body weight. The slow-death factors may also contribute to the toxicity of waterblooms. It is concluded that a complex of interdepedent variables determines the degree and kind of toxicity that a waterbloom can develop.