Lunasin is a novel peptide originally identified in soybean that suppresses chemical carcinogen-induced transformation in mammalian cells and skin carcinogenesis in mice. Since the lunasin gene was cloned from soybean and the chemically synthesized form of the lunasin peptide has been used in experiments conducted so far, the isolation of lunasin from other natural sources and testing of its biological properties have not been carried out. We report here the isolation, purification, and biological assay of lunasin from barley, a newly found rich source of the peptide. The identity of lunasin was established by Western blot analysis and mass spectrometric peptide mapping of the in-gel tryptic digest of the putative protein band. Lunasin was partially purified with anion exchange and immunoaffinity chromatography. The crude and partially purified lunasin from barley suppressed colony formation in stably ras-transfected mouse fibroblast cells induced with IPTG. These fractions also inhibited histone acetylation in mouse fibroblast NIH 3T3 and human breast MCF-7 cells in the presence of the histone deacetylase inhibitor sodium butyrate.