The amino acid sequence of the 27 kDa protein responsible for the haemolytic activity of Bacillus thuringiensis subsp. israelensis toxin has been analysed by secondary structure prediction, helical wheel/net diagrams and molecular mechanics calculations. We found that segment 116-126 presumably forms a strongly amphiphilic alpha-helix. This is supported by the findings that the synthesized segment 116-126 (a) has a significant alpha-helical content in water, and (b) displays an in vitro haemolytic activity comparable to that of bee venom peptide melittin. As segment 116-126 is present in the haemolyzing, but not present in the non-haemolyzing proteins from B. thuringiensis toxins, we suggest that this segment is responsible for the lytic potential of the B. thuringiensis subsp. israelensis protein.