Two extracellular isoenzymes of polygalacturonase, isolated from the brown-rot Fungus Postia placenta. were purified 342-fold by Mono S cation-exchange chromatography. The temperature optimum ranged from 25 degrees C to 37 degrees C, and the pH optimum ranged from 3.2 to 3.9. Apparent pI values of the isoenzymes (3.2 and 3.4) were lower than any previously reported. The estimated molecular mass from a single band on sodium dodecyl sulfate/polyacrylamide gel electrophoresis (PAGE) was 34 kDa. Isoenzymes of polygalacturonase in native PAGE and isoelectric focusing gels were identified by substrate/agar overlays (zymograms). Comparison of viscosity reduction rates with release of reducing sugars indicated that the enzyme from P. placenta is endo-acting. The objective of this study was to isolate polygalacturonase from the brown-rot Fungus P. placenta and characterize the properties of the enzyme.