When B. cinerea was grown on a modified Czapek Dox liquid medium containing galacturonic acid or glucose, there was no consistent increase with time in the total activity of extracellular polygalacturonase (PG), relative to mycelial dry weight. However, preparative isoelectric focusing identified a peak of activity between pH 3.7 and 6.3 only when the fungus was grown in the presence of galacturonic acid. This activity was attributed to the induction of exo-PGs (exo-PGI and/or exo-PGII). Antisera raised against 1 of 2 endo-PGs (endo PGI) reacted positively and specifically with endo-PGI and endo-PGII on Western blots, and both of these isoenzymes were present in Western blots when the fungus was grown in the presence of glucose or galacturonic acid after 48 h in vitro. Plate-trapped antigen-ELISA confirmed that the endo-PGs were constitutively expressed.