Cell walls of Microcystis sp. PCC 7806 were purified from cell homogenates by sucrose density centrifugation and Triton X-100 extraction. The outer membrane contained carotenoids, two major peptidoglycan-associated proteins (Mr 49,000 and 52,000), and lipopolysaccharide (LPS) as indicated by the presence of 3-hydroxy fatty acids (3-OH-14:0, 3-OH-16:0, 3-OH-18:0), 4-oxo-18:0 fatty acid, and GlcN as lipid A components in addition to rare O-methyl sugars (2-O-methyl-6-deoxyhexoses I and II). The peptidoglycan (A1 gamma-type) was found to be covalently linked to a wall polysaccharide composed of GlcN, ManN, Man, Glc, and phosphate.