Human apohemoglobin was found to induce fusion of various vesicles containing phosphatidylserine at low pH. The fusion of vesicles by apoHb was monitored by intermixing of internal aqueous contents, the resonance energy transfer assay which follows the mixing of membrane components and size increase from the electron microscopy and light scattering measurement. The fusion-pH profiles were similar to the binding profiles of apoHb to the same vesicles. Proteolytic digestion after hydrophobic labeling of PS and PS/PE vesicle-protein complex with dansyl chloride showed that a segment with a molecular weight of approximately 2500 penetrates the bilayer. It was also found that PS/PC(1:1) vesicles not only induces fusion of PS/PC(1:1) vesicles at low protein concentration but also frament the same vesicles to form micellar complex at high protein concentration. The micellization of PS/PC(1:1) vesicles was confirmed by light scattering, gel filtration and electron microscopy. The [125I] TID-labeling of the apohemoglobin in the vesicle-protein complex followed by CNBr cleavage of apohemoglobin showed that an N-terminal segment of $\beta$ subunit with a molecular weight of approximately 6000 seems to be mainly involved in the fusion process but the whole sequences of both a and $\beta$ chains participate in the micellization process. These results suggest thatn apoHb assumes differet topologies in the vesicle-protein complex depending on the L/P ratio causing either fusion or fragmentation of vesicles.