Objective: Hemolysin II (HlyII) is one of the key pathogenic factors of the opportunistic gram-positive bacterium Bacillus cereus. HlyII lyses target cells by forming pores on membranes. HlyII belongs to the group of β-pore-forming toxins. A distinctive feature of HlyII is the presence of a C-terminal domain of 94 amino acid residues (HlyIICTD). It was shown that, under slightly acidic conditions (pH 5.0) corresponding to the perimembrane region, the C-terminal domains, both by themselves and as part of the toxin, form stable complexes consisting of full-length and truncated toxin molecules. Methods: HlyII, HlyIILCTD (large C-terminal fragment Met225–Ile412), and HlyIICTD were obtained using recombinant producer strains Escherichia coli BL21(DE3). Biotinylation of HlyIICTD was carried out using N-hydroxysuccinimide ester of biotin. The interaction of HlyIICTD with HlyIICTD, HlyIILCTD, and HlyII, as well as that of HlyIICTD with erythrocyte membranes, were studied by enzyme-linked immunosorbent assay and immunoblotting using both horseradish peroxidase-conjugated streptavidin and monoclonal antibodies against HlyII. Results and Discussion: Under slightly acidic conditions, HlyIICTD interacted with both the HlyIICTD domain within the full-length toxin and with the HlyIICTD protein. The interaction of HlyIICTD with the erythrocyte membrane was enhanced several-fold in the presence of the toxin. Conclusions: The property of the C-terminal domain to form complexes with other HlyIICTDs, regardless of whether it is part of the full-length toxin, the large C-terminal fragment, or the short HlyIICTD under conditions corresponding to those existing near the cell membrane (pH 5.0), was revealed. The toxin in the perimembrane region exists in a partially molten-globule state, in which the C-terminal domains of the monomers can bind to each other, increasing the local concentration of full-length toxins.