A novel polypeptide (Mere15) was purified from Meretrix meretrix Linnaeus by ammonium sulfate fractionation, ion exchange, gel filtration and reversed phase chromatography. Mere15 exhibited selective cytotoxicity to several human cancer cells. In vivo study showed that Mere15 significantly suppressed the growth of human lung adenocarcinoma A549 xenograft in nude mice. The mechanism was associated with a G(2)/M phase arrest followed by apoptosis, including membrane blebbing, loss of mitochondrial membrane potential, externalization of phosphatidylserine, chromosome condensation and DNA fragmentation. Western blot analysis showed that the intrinsic pathway was involved in Mere15-induced apoptosis. These results suggest that Mere15 may have therapeutic potential for the treatment of non-small-cell lung carcinoma.