Some of the isoperoxidases present in an extract from zucchini hypocotyls--one anionic and two cationic--exhibited a Ca²⁺-dependent pelletability which resulted from an interaction with pectins. Endogenous pectins could be replaced by polygalacturonic acid or pectin extracted from citrus, but not by highly esterified pectin. The interaction between the isoperoxidases and the polysaccharides has been studied by centrifugation, by gel filtration, and with pectins attached in wells of a microtitration plate. These various binding tests have shown that the isoperoxidases had an affinity for the pectins in their Ca²⁺-induced conformation.