The endotoxin from the blue–green alga Microcystis aeruginosa NRC-1 has been isolated as its ammonium salt by a new procedure involving solvent extraction and elution from DEAE-Sephadex A-25. The intraperitoneal LD100 of this toxin preparation for mice was 0.1 mg/kg body weight. The product is a white solid which appears to be chromatographically and electrophoretically homogeneous. This toxin produces similar symptoms in mice to those described originally for the toxic material from the same algal strain. Analysis of the toxin hydrolysate indicates a more complex structure since seven more amino acids have been found, including tyrosine, proline, and arginine, in addition to the seven reported before.