Subtilisin inhibitors (SI) having an average molecular mass of 8 kDa purified from jack beans (Canavalia ensiformis) and broad beans (Vicia faba) to electrophoretical homogeneity are compared to those isolated from chick peas (Cicer arietinum) and black beans (Phaseolus vulgaris). The specificity spectrum of SI is restricted to microbial serine proteinases. Their interaction with subtilisin Carlsberg is variable, obtaining the highest value for black bean, followed by broad bean, chick pea and jack bean inhibitors. Proteinase K reacts with most SI twice as strongly as subtilisin. The reaction of SI with subtilisin apparently follows the "standard mechanism" proposed by Laskowski for the interaction of trypsin with its inhibitors. Accordingly, the reactive site of jack bean, broad bean and black bean SI is reversibly cleaved by subtilisin in acid and neutral solutions, whereas chick pea SI is hydrolysed only at neutral pH values. Judged by similarities in specificity, molecular mass, heat stability and reactive site cleavage, the SI from legume seeds could constitute a new "family" of proteinase inhibitors.