Blooms of the blue-green alga, Microcystis aeruginosa, consisting almost wholly of this organism were tested for toxicity, principally with mice but also with guinea pigs and rabbits. The fresh material given parenterally was toxic to a limited degree, but was non-toxic when given by mouth. Water from the algae, and other micro-organisms from the algal collections were non-toxic. Frozen material, and frozen material dried in vacuo were highly toxic when administered parenterally. Attempts were made to extract the toxic agent, but an insufficient amt. was secured to definitely determine many chemical characteristics. It is dialysable, water- and alcohol-soluble, adsorbable on charcoal, and heat stable in neutral soln. It does not appear to be of a bacterial nature. Three other blue-green algae were non-toxic.