In the presence of the Cu(I)-chelating agent neocuproine (2,10-dimethyl-1,9-phenanthroline) hydrogen peroxide acta as a reductant of Cu(II). The reaction does not proceed in the absence of neocuproine and the addition of EDTA to the reaction mixture prior to addition of Cu(II) also inhibits the reduction. Colour development can be arrested and stabilized by addition of EDTA. The reaction can be used to estimate hydrogen peroxide concentrations in the range 0.68-6.8 μg/ml (20–200 μM). Horseradish peroxidase is not required for the peroxide assay by glucose oxidase must be used for glucose estimations. Thermostable cellulase activity has been estimated at 60°C against cellobiose, carboxymethylcellulose substrates by estimation of the glucose released from the substrates.