Interaction of melittin with lipid membranes was studied systematically with respect to its adsorption onto membranes, its effect on membrane leakage and fusion, and micellization at various melittin/lipid ratios. It was found that melittin has a strong affinity for adsorption onto lipid membranes. The analysis of the measured electrophoretic mobilities by use of a Gouy-Chapman double layer theory, shows that melittin is adsorbed onto the phosphatidylserine membrane several times more than the phosphatidylcholine membrane. However, it was observed that the phosphatidylcholine membrane is more susceptible to membrane leakage, vesicle fusion and micellization at a lower level of melittin adsorbed than the phosphatidylserine membrane. For small unilamellar phosphatidylcholine vesicles in 0.1 M NaCl, membrane leakage started at melittin to lipid ratio of 1:2000, a large increase in the rate of membrane leakage occurred at a ratio of about 1:500 or higher, membrane fusion occurred at a ratio of 1:200, and membrane micellization at a ratio of 1:10. On the other hand, for small unilamellar phosphatidylserine vesicles, the respective concentrations of melittin to result in membrane leakage, vesicle fusion, and membrane micellization were several times higher. Surface pressure measurements of lipid monolayers showed that the increase in surface pressure of the phosphatidylcholine monolayer due to the presence of melittin in the subphase solution was greater than that for the phosphatidylserine monolayer at any melittin concentration in the subphase solution. These experimental results indicate that melittin tends to be adsorbed on the surface of the negatively charged phosphatidylserine membrane due to the electrostatic binding so that the melittin molecule can stay out more on the surface of the membrane, while melittin appears to be adsorbed more into the hydrophobic membrane core for the electrically neutral phosphatidylcholine membrane.