Lipid compositions of cell membrane of Mycoplasma orale CH 19299 were examined. The membrane lipids consisted of neutral lipids (43.5%) and phospholipids (56.5%). In the neutral lipid, 64% was cholesterol, 9% was cholesterol ester and the rest mainly consisted of triglycerides. Phospholipids comprize 53% of sphingomyelin, 32% of phosphatidylglycerol and 15% of phosphatidylcholine. There was not any lysophosphatidic acid, cardiolipin or glycosyldiglycerides. The fatty acids were restricted to C(16) and C(18) regardless of the phospholipid species. Phosphatidylglycerol was the sole phospholipid that was de novo synthesized in the cell. No fatty acid was synthesized when examined by using (14)C-acetate as the substrate. The mode of lipid synthesis in the cell membrane was discussed with reference to other mycoplasma species.