The presence of lunasin was investigated in two soybean products, raw soy seeds and a commercial soybean beverage powder. Lunasin from the commercial soybean derivative was purified by reverse phase high pressure liquid chromatography and bystrong anion exchange solid phase extraction. Lunasin was further characterized by Top-down mass spectrometry (MS). The top-down characterization of lunasin unraveled a wide range of post-translationally modified proteoforms.