ABSTRACTThe two major trypsin‐chymotrypsin inhibitors ofLens culinarisMedik., LCI‐1 and LCI‐4, were purified to homogeneity from Italian red lentils by ammonium sulfate fractionation, gel and ion‐exchange chromatography. At least two more trypsin‐chymotrypsin inhibitors of minor importance were present. LCI‐1 and LCI‐4 inhibited human trypsin less (68–74%) but human chymotrypsin better (268–279%) than the respective bovine enzymes. The two inhibitors contained no carbohydrates, no methionine and no tryptophan and high contents ofcystine but no free sulfhydryl groups (seven and eight disulfide bridges for LCI‐1 and LCI‐4, respectively). In addition, LCI‐4 contained no isoleucine. The isoelectric points were 5.35 and 7.70 and the average molecular weights 10,600 and 9,900 daltons for LCI‐1 and LCI‐4, respectively. Inhibitor extracts, as well as purified inhibitors in solution, were relatively stable against heating and treatment with human gastric juice, while soaking the whole seeds overnight and subsequent boiling for two hours totally d