The interaction of $\alpha$ -LA with large unilamellar vehicles of PS and PS/PE (1:1) were studied. $\alpha$ -LA was found to induce fusion of PS and PS/PE vesicles at low pH. Its fusogenic behavior and binding capacity to these two vesicle systems were compared at a wide range of pH. The fusogenic behaviors of $\alpha$ -LA in these two vesicle systems were similar. At pH 3, 4 and 5, fusion was observed at almost same quantity in these two systems. But, the binding capacities were somewhat different. The amount of protein bound to PS vesicles was greater than that of PS/PE vesicles. $\alpha$ -LA influenced the spin labels deep inside the bilayer of PS vesicles while only one near the surface was affected for the case of PS/PE vesicles. Hydrophobic labeling experiment with photoreactive phosphatidylcholines showed same result.