Polyhistidine facilities the fusion of negatively charged liposomes prepared by sonication. Liposome fusion was demonstrated by (a) negative-stain electron microscopy, (b) gel filtration, and (c) resonance energy transfer of the fluorescent phospholipids. Liposome fusion required the presence of polyhistidine, whereas histidine at equivalent concentrations had no effect. Little or no liposome fusion was detectable at pH 7.4, but it was greatly enhanced when the pH of the medium was reduced below 6.5. Although acidic phospholipid is necessary for fusion, liposomes made of acidic lipids alone showed only low levels of fusion activity. Liposomes composed of mixtures (1:1) of a negatively charged phospholipid and a neutral phospholipid such as phosphatidylcholine and phosphatidylethanolamine (PE), but not sphingomyelin, showed high levels (about 80%) of fusion competency. For liposomes made of PE/phosphatidylserine (PS) (1:1), fusion at pH 5.2 and 2.5 micrograms/mL polyhistidine resulted in an increase in the average liposome diameter from 296 to 2400 A, indicating multiple rounds of fusion had occurred. Liposome fusion was not very leaky as revealed by the lack of release of encapsulated calcein. For PE/PS (1:1) liposomes, about 10% of dye leakage was observed for up to about 30% liposome fusion and about 45% leakage at 80% liposome fusion. Since polyhistidine becomes a strong polycation at acidic pH, liposome fusion may be a direct result of the bilayer phase separation induced by the binding of polyhistidine with the negatively charged phospholipids. Therefore, this phenomenon is similar to the liposome fusion induced by other polycations such as polylysine at neutral pH.(ABSTRACT TRUNCATED AT 250 WORDS)