Cochinin B, a novel ribosome-inactivating protein (RIP) with a molecular weight of 28 kDa, was purified from the seeds of Momordica cochinchinensis (Cucurbitaceae). The isolation procedure entailed ammonium sulfate precipitation, cation-exchange chromatography on SP Sepharose column and size-exclusion chromatography on Superdex 75 column with a fast protein liquid chromatography (FPLC) system. The first twenty N-terminal amino acid residues of Cochinin B showed homology to type I RIPs from other Momordica species. The purified Cochinin B displayed a strong inhibitory activity on protein synthesis in the cell-free rabbit reticulocyte lysate system with IC50 of 0.36 nM. Furthermore, it exhibited N-glycosidase activity and cytotoxicity against Vero cell line with IC50 higher than 1540 nM. Interestingly, Cochinin B manifested strong anti-tumor activities on human cervical epithelial carcinoma (HeLa), human embryonic kidney (HEK293) and human small cell lung cancer (NCI-H187) cell lines with IC50 of 16.9, 114 and 574 nM, respectively.