The electrophoretically homogeneous protein obtained from the culture filtrate of Stereum purpureum causes silver-leaf symptoms when injected into apple trees at a dosage of about 25 μg per tree. This preparation has a molecular weight of 32, 000, and shows not only an endo-PG (polygalacturonase) activity, but also macerating action in several plant tissues. Toxic effects on apple trees qualitatively make parallel alteration in the endo-PG activity. From the chemical properties tested, it is suggested that the preparation differs from phytolysin reported as a toxic material inductive to silver-leaf symptoms on plum trees.