In Acholeplasma laidlawii A, strain EF 22, the relative amounts of the membrane polar lipids vary as a consequence of different fatty acid supplements to the growth medium. The number of lipid species also varies; a new apolar monoglucolipid containing four fatty acid residues was present only when saturated fatty acids dominated in the growth medium. A new phosphoglucolipid, probably with a glycerophosphoryl-monoglucosyldiglyceride structure, was also found. The most pronounced variations occurred between the two dominating glucolipids, monoglucosyldiglyceride and diglucosyldiglyceride; the former being found in larger amounts when a saturated or a trans-unsaturated fatty acid was present in the medium. The amount of diglucosyldiglyceride decreased accordingly. A qualitative relationship between fatty acid properties and membrane lipid variations was established over a wide fatty acid concentration range. Incorporation of supplied fatty acids reached higher levels than normally found in other acholeplasmas. The ratio between membrane protein and lipids exhibited significant and coherent variations during growth and was to some extent influenced by the fatty acids in the medium. These changes indicate variations in lipid-protein organization in the membranes during growth.