Abstract (1) By treating Mycoplasma capricolum cells with phospholipase A 2 about 80% of membrane phospholipids were rapidly hydrolyzed. The rate and extent of hydrolysis (at 37°C) were the same in intact cells and in isolated unsealed membranes. (2) Due to the low endogenous lysophospholipase activity detected in M. capricolum, phospholipase A 2 treatment resulted in the accumulation of lysophospholipids and free fatty acids. The free fatty acids were efficiently extracted from the cells by 1% bovine serum albumin whereas the lysophospholipids were almost fully retained within the cell membrane. (3) Following phospholipase A 2 treatment in the presence of 1% bovine serum albumin, cell intactness was preserved as indicated by the constant absorbance of the cell suspension and the retention of nucleic acids and NADH dehydrogenase activity within the cells. The treated cells showed, however, a slight decrease in K + content and a decrease in cell viability. Viability was fully preserved after phospholipase A 2 treatment of cells grown with exogenous sphingomyelin. (4) Adapting M. capricolum to a cholesterol-poor medium resulted in a marked decrease in the cholesterol to phospholipid molar ratio (from about 1.1 to 0.3). Phospholipase A 2 treatment of the cholesterol-poor cells resuted in cell lysis. Cell lysis was induced in the cholesterol-rich cells by hydrolysing the lysophospholipids accumulated following phospholipase A 2 treatment. (5) It is suggested that after phospholipase A 2 treatment of M. capricolum cells, a relatively stable cell membrane is maintained and cell intactness is preseved due to the interaction of cholesterol, present in high amount in this membrane, with the lysophospholipids formed.