Four Secale cereale L. xylanase inhibitors (SCXI) from rye were purified to homogeneity. The inhibitor proteins all occur in two molecular forms, i.e. a c. 40kDa monomeric protein and, presumably following proteolytic modification, a heterodimer consisting of two disulphide linked subunits of c. 30 and c. 10kDa. These basic proteins all have isoelectric points of at least 9.0 and a highly homologous N -terminal amino acid sequence. The isolated proteins strongly inhibited the activity of Aspergillus niger, Bacillus subtilis and Trichoderma viride family 11 endoxylanases, but showed no activity towards an Aspergillus aculeatus family 10 endoxylanase. These characteristics demonstrate that rye contains a family of isoinhibitors with similar structures and specificities, that are homologous with Triticum aestivum L. xylanase inhibitor I (TAXI I) from wheat andHordeum vulgare L. xylanase inhibitor (HVXI) from barley.