A trypsin inhibitor with a Km of 5 x 10(-5) M has been isolated from kohlrabi (Brassica napus var. rapifera). Subtilisin DY is inhibited only weakly and chymotrypsin not at all. The inhibitor is closely related to napin as determined by amino acid sequence analysis which also showed the inhibitor to be polymorphous. The inhibitor has been further characterized by means of molecular weight determination using SDS gel-electrophoresis and by amino acid analysis, fluorimetry as well as circular dichroism. A simplified method for purification of napins is given.