The standard filter paper assay (FPA) published by the International Union of Pure and Applied Chemistry (IUPAC) is widely used to determine total cellulase activity. However, the IUPAC method is not suitable for the parallel analyses of large sample numbers. We describe here a microplate-based method for assaying large sample numbers. To achieve this, we reduced the enzymatic reaction volume to 60 mul from the 1.5 ml used in the IUPAC method. The modified 60-mul format FPA can be carried out in 96-well assay plates. Statistical analyses showed that the cellulase activities of commercial cellulases from Trichoderma reesei and Aspergillus species determined with our 60-mul format FPA were not significantly different from the activities measured with the standard FPA. Our results also indicate that the 60-mul format FPA is quantitative and highly reproducible. Moreover, the addition of excess beta-glucosidase increased the sensitivity of the assay by up to 60%. (C) 2004 Wiley Periodicals, Inc.