The rumen anaerobic fungus Piromonas communis, unlike the rumen anaerobic fungi Neocallimastix frontalis and Neocallimastix patriciarum, produced extracellular alpha-(4-O-methyl)-D-glucuronidase when grown in cultures containing filter-paper, barley straw, birchwood xylan or birchwood sawdust as carbon source. The highest concentration of enzyme was produced in cultures containing birchwood sawdust. The aldobiouronic acid O-alpha-(4-O-methyl-D-glucopyranosyluronic acid)-(1 --> 2)-D-xylopyranose (MeGlcAXyl) was the best substrate of those tested : the aldotriouronic acid O-alpha-(4-O-methyl-D-glucopyranosyluronic acid (1 --> 2)-O-beta-D-xylopyranosyl-(1 --> 4)-D-xylopyranose (MeGlcAXyl(2)) and the aldotetraouronic acid O-alpha-(4-O-methyl-D-glucopyranosyluronic acid)-(1 --> 2)-O-beta-D-xylopyranosyl-(1 --> 4)-O-beta-D-xylopyranosyl-(1 --> 4)-D-xylopyranose (MeGlcAXyl(3)) were also attacked but the rate fell as the degree of polymerisation increased. When the same substituted xylooligosaccharides were reduced to the corresponding alditols the enzyme activity disappeared. Similarly, p-nitrophenyl-alpha-D-glucuronide was not a substrate. Remarkably, the relative rates of attack shown by the alpha-(4-O-methyl)-D-glucuronidase on the aldouronic acids and on xylans extracted from birchwood, oat spelts and oat straw differed according to the carbon source used to produce the enzyme. The alpha-(4-O-methyl)-D-glucuronidase had a pH optimum of 5.5 and a temperature optimum of 50 degrees C. On gel filtration the enzyme was shown to be associated with proteins covering the range 100-300 kDa, but a major peak of activity in the column effluent appeared to have a molecular mass of 103 kDa.