Lipopolysaccharides (LPS) were isolated from four species of cyanobacteria (Anabaena flos-aquae UTEX 1444. A. cylindrica, Oscillatoria tenuis, and O. brevis) frequently occurring in drinking-water supplies. The cyanobacterial LPS contained glucose, xylose, mannose, and rhamnose, but differed from the LPS derived from most gram-negative bacteria because of the variable presence of 2-keto-3-deoxyoctonate, heptose, galactose, and glucosamine. Cyanobacterial lipid A is characterized by long-chain saturated an unsaturated fatty acids and hydroxy fatty acids which show great diversity. Unlike lipid A from heterotrophic gram-negative bacteria, lipid A from cyanobacteria usually lacks phosphates. The detection of distinct O-antigen chemotypes indicates that LPS may be used for taxonomic classification. Isolated cyanobacterial LPS always induced Limulus amoebocyte lysate gelation. A. flos-aquae LPS gave a positive Schwartzman reaction. Endotoxins from A. cylindrica and O. brevis were toxic to mice when injected intraperitoneally. The cyanobacterial endotoxins showed generally lower biological activity than did LPS derived from common heterotrophic gram-negative bacteria. Nevertheless, cyanobacteria in algal blooms may be a significant source of endotoxins in water supplies.