The effects of removing the cell wall on the lipid composition of the plasma membrane of Gram-negative Ps. aeruginosa were examined, and compared with the effects observed with Gram-positive bacteria like S. aureus (Hayami et. al, Microbiol. Immunol. 23(6), 1979). The L-form, which was derived from Ps. aeruginosa IFO 3455 by treatment with EDTA-lysozyme, was comprised of 71.5% phospholipids(PL), 5.3% glycolipids(GL) and 23.2% neutral lipids(NL). The phospholipids included phosphatidylethanolamine(PE), cardiolipin(CL), phosphatidylglycerol(PG), and phosphatidylcholine(PC) at the relative concentrations of 53, 27, 12, and 5% , respectively. In comparison with the bacterial form, neutral lipids and CL content was higher in the L-form, but PE was lower. Fatty acid analysis showed that the L-form contained more C(16) saturated fatty acid than the bacterial form. The L-form also contained the C(17cyc') C(19cyc') and C(18-OH) types of fatty acids which were not detected in the bacterial form. These findings are similar to those obtained with S. aureus, indicating that the phospholipids and fatty acids in the plasma membrane play an important role in adaptational changes or compensation for the loss of the cell wall.