Protein proteinase inhibitors are widely distributed in plant seeds, particularly in legumes. The specificity and potency of inhibition depend on defined inhibitory sites and on the animal species of the target proteinase. Feeding experiments on diets containing isolated soybean trypsin inhibitors (the Kunitz soybean trypsin inhibitor STI and the Bowman-Birk trypsinchymotrypsin inhibitor BBI) caused insignificant growth depression in rats and chicks, but induced enlargement of the pancreas in rats, chicks and mice but not in pigs, dogs, calves, monkeys and presumably humans. The trypsin-inhibitory site has been responsible for induction of the pancreatic enlargement. The trypsin-chymotrypsin inhibitors from soybeans and from chickpeas inhibit insect midgut proteinases, supporting the hypothesis that proteinase inhibitors comprise a built-in defense mechanism of the seed against insects. Findings on the involvement of proteinase inhibitors, such as BBI, in prevention of tumorigenesis suggest a possible positive contribution of the inhibitors to the nutritional value of legume seeds. BBI is also an effective inhibitor of nephrotoxicity induced by the antibiotic gentamicin. BBI does not cause side effects and does affect the antimicrobial activity. The in vitro effects of proteinase inhibitors on animals should be interpreted with caution when related to humans.