High-throughput screening requires simple assays that give reliable quantitative results. A microplate assay was developed for reducing sugar analysis that uses a 2,2'-bicinchoninic-based protein reagent. Endo-1,4-beta-D-xylanase activity against oat spelt xylan was detected at activities of 0.002 to 0.011 IU ml(-1). The assay is linear for sugar concentrations from 0 to 86 mug ml(-1) and can also be used to assay protein concentrations (0 to 143 mug ml(-1)) on the same plate. A variety of temperatures and pH conditions can be used and, after incubation, the assay requires only one detection reagent and one heating step.