The endopolygalacturonase (endo-PG) inhibitor from Pisum sativum (pea) was characterized in terms of its effect on PGs from five pathotypes of Ascochyta pisi and seven other species of fungi and its occurrence in 14 pea varieties and species from three other genera. Each pathotype of Ascochyta pisi produced both an exo- and endo-PG and, in each case, the latter enzyme only was affected by the endo-PG inhibitor. Although each of the three other pea pathogens examined here produced an endo-PG, none of these enzymes was sensitive to the endo-PG inhibitor. Of the remaining four fungi, only Phoma solani produced exclusively exo-PG and Colletotrichum lindemuthianum and Aspergillus niger produced endo-PGs that were sensitive to the endo-PG inhibitor from pea. The endo-PG inhibitor occurred at highest concentration in the stipules and leaflets of the pea, and more than 75% was recovered in a soluble form in extracts of these organs made in 50 mm sodium acetate buffer (pH 5·0). Inhibitor was detected in each pea variety tested, although the yield varied considerably. Inhibitor of Ascochyta pisi endo-PG was similarly extracted from hypocotyls of Phaseolus vulgaris (bean), but was not detected in extracts of cucumber and cabbage seedlings. These results indicate that the endo-PG inhibitor is not directly responsible for the differential resistance of pea varieties to Ascochyta pisi pathovars. In addition, they provide further evidence of similarities between the endo-PGs of C. lindemuthianum and Ascochyta pisi and between the endo-PG inhibitors from pea and bean.