Single-walled asolectin vesicles loaded with K+ at pH 7.00 released their K+ content upon incubation with tetanus toxin fragment B but only when the incubation was at pH below 5.00. Whole tetanus toxin exhibited only a weak releasing activity. Toxin light alpha chain and the carboxyl-terminal 48,000-dalton moiety of the heavy chain (fragment IIC) were unable to provoke K+ release from vesicles at any pH. K+ release from lipid vesicles could also be detected with tetanus toxin heavy beta chain at low pH. Furthermore, using a detergent binding assay ([3H]Triton X-100), we have also shown that an hydrophobic domain, localized in the 50,000-dalton terminal polypeptide of tetanus toxin heavy chain, is detectable at pH 3.60 but not at pH 5.00. These results lead us to conclude that the ability of tetanus toxin fragment B to release K+ from asolectin vesicles at low pH is due to the 50,000-dalton amino-terminal polypeptide of the heavy chain present in toxin fragment B. We propose that this phenomenon is caused by channel formation across the vesicle membrane as has been observed for the 23,000-dalton amino-terminal moiety of diphtheria toxin fragment B.