Cry51Aa1, a novel crystal protein comprised of 309 amino acid residues with a predicted molecular weight of 34 kDa, was purified from Bacillus thuringiensis strain F14-1. After trypsin digestion, we observed three clear smaller bands of Cry51Aa1 protein upon separation by gel. In addition, this digested Cry51Aa1 protein had no obvious toxicity to several types of insect larvae and hemolytic activity human erythrocyte. However, the activated Cry51Aa1 could exhibit cytocydal activity against to MDA-MB-231, HCT116, CaCo2 and U87 tumor cells, while no cytotoxic activity was found in Cry51Aa1 treated PC3, HepG2 and LO2 cells. Importantly, MDA-MB-231 cells showed the most sensitivity to activated Cry51Aa1 within all the cell lines assessed. These observations referred to the presence of a new non insecticidal toxin of Cry51Aa1, which should be a new parasporin protein.