A serine protease, ingobsin, that cleaves Lys-x and Arg-x, has been purified from rat duodenal tissue. By immunohistochemical methods, the enzyme was localized in goblet cells in the small intestine of rat, pig, and man. The immunoreactive cells were most numerous in the proximal part of the intestine. In the electron microscope, the immunoreaction was localized mainly to the rough endoplasmic reticulum of the goblet cells and to the secretion being extruded from the cells.