Lipopolysaccharide (LPS) fractions were isolated from three species of blue-green algae of the genus Phormidium, namely, P. africanum, P. laminosum, and P. uncinatum, by using a phenol-water procedure followed by exhaustive extraction with ammonium oxalate. The materials obtained were shown to be closely related biochemically. Nearly 60% of the LPS consisted of the polysaccharides galactose, glucose, mannose, xylose, arabinose, and rhamnose and an unidentified, fast-moving sugar residue. In addition, glucosamine, galactosamine, and 2-keto-3-deoxyoctonate were detected. Oleic, stearic, and palmitic acids were found in the hydrolysate of the lipid component, which averaged 1.5% of the LPS. Concomitantly, the protein component (7 to 20%) was shown to contain the following amino acids: aspartic acid, threonine, serine glutamic acid, proline, glycine, alanine, valine, methionine, isoleucine, leucine, tyrosine, phenylalanine, lysine, histidine, and arginine. Whole cells, as well as the LPS, of Phormidium possessed antigenic properties.