To clarify salt-resistant mechanism of Staphylococcus aureus from the standpoint of the passive transport, attempts were made to establish the experimental system of artificial model membrane, liposome, and to measure permeability of Na(+) to it. The liposomes were prepared from phospholipids of S. aureus cultivated in normal and 10% NaCl media and NaCl-sensitive mutants. 1. In physiological conditions, it is possible to form the stable liposome comprised of phospholipids or total lipids of S. aureus. The preparation methods give a remarkable difference in the shape of vesicles. In the case of particles produced by hand or mechanical shaking, the diversities in the size and shape of individual particles can be observed. Sonicator-prepared liposomes are small and homogeneous of particle size, and consist of unilamella rather than multilamella. 2. Salt concentration affects liposome formation. Cardiolipin containing abundant branchedfatty acid form stable vesicles even in a high concentration of NaCl solution. 3. The liposome prepared from phospholipids of 10% NaCl S. aureus suppresses more markedly in the rate of Na(+) diffusion than that of normal S. aureus. The experiments on the liposomes prepared from fractionated individual phospholipids indicate that cardiolipin containing branchedfatty acid contributes to salt-resistance of S. aureus as a barrier. 4. From optical measurements, it has been elucidated that the liposome of phospholipids prepared from S. aureus acts as an ideal osmometer, and non-electrolyte, glycerol, shows a similar behavior to Na(+) in permeability.