Mycoplasma fermentans is thought to be a pathogen of rheumatoid arthritis or a cofactor of AIDS (acquired immunodeficiency syndrome). To elucidate the possible involvement of membrane constituents in the pathogenesis of these diseases, we studied its lipid components. Several alkali labile glycophospholipids were detected and named glycoglycerophospholipids (GGPLs). Previously, we purified and determined the structure of one of them as 6'-O-phosphocholine-alpha-glucopyranosyl-(1'-3)-1,2-diacyl-sn-glycerol (GGPL-I). The present paper describes the purification and structural characterization of GGPL-III, the major GGPL of M. fermentans using 1H-, 13C- and 31P-nuclear magnetic resonance spectroscopy, and mass-spectroscopy as 1"-phosphocholine,2"-amino dihydroxypropane-3"-phospho-6'-alpha-glucopyranosyl-(1'-3)-1,2-dia cyl-glycerol.