The activities of 4 fungal polygalacturonases (endo-PGI, endo-PGII, exo-PGI and exo-PGII), detected when B. cinerea was grown on immature fruits of red raspberry, were fractionated into soluble and wall-bound fractions. Western blots and plate-trapped antigen ELISA showed that endo-PGI and endo-PGII were most abundant in the cell wallbound fractions of the host. Immuno-inhibition studies using a polyclonal antiserum against polygalacturonase-inhibiting protein (PGIP), purified from immature raspberry fruits, showed that the low level of fungal PG activity detected in fractions containing endo-PGI was due to the presence of PGIP. When a purified preparation of endo-PGI and endo-PGII from B. cinerea was allowed to react in vitro with either a crude host cell wall preparation, or one which had previously been treated to remove cell wall-bound proteins, both endo-PG isoenzymes had a greater binding capacity towards the former wall preparation. Endo-PGI and endo-PGII also had an affinity for fungal cell walls. Exo-PGI and exo-PGII bound to both fungal and host cell walls. Greater quantities of fungal endo-PGs were detected by ELISA in fruits previously frozen and thawed ('freeze-thawed') and inoculated, than in fresh inoculated fruit. This result paralleled the extent of fungal growth in these tissues as assessed by chitin assay and it is suggested that the resistance shown by raspberries is dependent on continual replacement of inhibitory substances or induced resistance mechanisms.