Polygalacturonase is extractable from ripe tomatoes in two isoenzyme forms, polygalacturonase 1 and 2. These isoenzymes have previously been shown to have substantially different properties although their polypeptides appear similar. Green fruit contain a heat-stable, non-dialysable factor capable of the conversion of polygalacturonase 2 in vitro into another isoenzyme which, on the basis of heat stability, molecular weight and density in caesium chloride, is equivalent to polygalacturonase 1. The amount of this factor extractable from tomato tissue increases during ripening.