A protein polygalacturonidase inhibitor isolated from fruit of the apple varieties Antonovka and Mantuanskoe differently affects the polygalacturonidases of different phytopathogenic fungi. Three groups of fungi were recognized by the sensitivity of their polygalacturonidases to the inhibitory effect. Storage of apples after harvesting is accompanied by changes in the inhibitor activity, and the time pattern of these changes depends on the variety. An increase in the inhibitor activity occurs concurrently with the elevation in ethylene release characteristic of the stage of elevated respiration (a climacteric increase). The data suggest that a decrease in the apple fruit resistance to microbes at the end of the storage period is related, along with other reasons, to a change in the activity of the protein polygalacturonidase inhibitor.