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Summary: Polygalacturonase (PG) was produced by Penicillium expansum in rotted apples and in liquid culture medium containing pectin; acetone precipitates of the enzyme were prepared from both sources. The PG extracted from apples (in vivo PG) behaved as an endo-PG and degraded sodium polypectate to tetragalacturonic acid. PG from culture filtrates (in vitro PG) before purification had quite different properties, the final hydrolysis product being mono-galacturonic acid. PG from both sources was purified by ion-exchange chromatography. The properties of some purified PG preparations were quite different from those of the crude material. None of the purified in vitro PG released monogalacturonic acid from sodium polypectate, whilst some preparations from both sources had identical endo-PG properties after purification. Changes in the properties of endo-PG produced in vivo and in vitro were apparent during purification and a series of distinct endo-PG’s were obtained which yielded unresolved oligomers, tri- or tetragalacturonic acid as final hydrolysis products. It is suggested that the changes in behaviour of the various PG preparations during purification were brought about by changes in the molecular configuration of the enzyme, and that such changes could account for some of the variability between enzymes produced in vivo and in vitro.