Agaricus bisporus H 25 produced extracellular endo-1, 3-beta-glucanase when grown in a static culture at 25 degrees C in a minimal synthetic medium supplemented with A. bisporus cell walls plus fructose. Endo-1,3-beta-glucanase was purified 17.85-fold from 20-day-old culture filtrates by precipitation at 80% ammonium sulfate saturation, Sephadex G-75 gel filtration, and preparative PAGE followed by electroelution. The purified enzyme yielded a single band in both native and SDS-polyacrylamide gels with a molecular mass of 32 kDa (SDS-PAGE) and 33.7 kDa (MALDI-MS), showing an isoelectric point of 3.7. The enzyme was active against beta-1,3- linkages and, to a lesser extent, against beta-1,6-, exhibiting an endohydrolytic mode of action and a glycoprotein nature. Significant activities of the endo-glucanase against laminarin and pustulan were observed between pH 4 and 5.5, and between 40 degrees and 50 degrees C for laminarin, and between 30 degrees and 50 degrees C for pustulan. The optimum pH and temperature were 4.5 and 45 degrees C for both substrates.