Cancer remains one of the biggest threats to human society. There are massive demands for compounds to selectively kill cancerous cells. Earlier studies have shown that bovine α -lactalbumin made lethal to tumor cells (BAMLET) becomes cytotoxic against cancer cells in complex with oleic acid {Hoque, M. et. al., PLoSOne 8, e68390 (2013)}. In our study, we obtained bovine α-lactalbumin complexed with lanthanum ion (La3+-B-α-LA) and determined its high resolution crystal structure. The natural calcium binding site of bovine α-lactalbumin is replaced by lanthanum. The La3+ complex formation by B-α-apo-LA was also supported by various biophysical methods. Interestingly, our complex, La3+-B-α-LA exhibits much greater anticancer activity against breast cancer cells as compared to the reported BAMLET-oleic acid complex. This study shows that La3+-B-α-LA complex is preferentially more toxic to MCF-7 cells as compared to KB (oral cancer) and HeLa (cervical) cells, while almost non-toxic to the healthy cells that we studied. Our data indicates that the cytotoxicity of La3+-B-α-LA against cancer cells is through apoptotic path way. The higher anticancer activity of La3+-B-α-LA is attributable to the requisite structural changes induced in the protein by La3+ binding as supported by the crystal structure of the complex.