The ability of the β-subunit of tomato endopolygalacturonase I (PGI) to enhance the thermal resistance of endopolygalacturonase II (PGII) has been tested after PGI was inactivated by thermal or manothermosonication (MTS) treatments of similar intensity. The β-subunit retained a low converter activity (0.3–1%) after thermal treatments whereas this activity was fully destroyed by MTS treatments. The low amount of PGI activity restored in the heat treated samples could be due to a low assay efficiency or to the thermal inactivation of the β-subunit.