An enzymatic end-point assay of trehalose using acid trehalase from yeast is described. After quantitative hydrolysis of trehalose by acid trehalase, the resulting glucose is assayed with the commercially available glucose oxidase/peroxidase dye system. Pre-existing glucose is determined in a control reaction from which acid trehalase is omitted. When intact cells are analysed for trehalose, pre-existing glucose can be washed out with ice-cold water without reducing the trehalose content of the cells. A convenient method for extraction of trehalose from intact yeast cells is heating for 20 min at 95 degrees C followed by centrifugation. The specificity of the assay is determined by the specificity of the acid trehalase preparation used. As described previously (Mittenbühler, K. and Holzer, H., 1988, J. Biol. Chem. 263, 8537-8543; Mittenbühler, K., 1988, Thesis, University of Freiburg), the following sugars and sugar derivatives do not form glucose when incubated with purified acid trehalase: sucrose, cellobiose, mellobiose, raffinose, maltose, lactose, glucose-6-phosphate, glucose-1-phosphate, galactose. The application of the new trehalose assay to yeast cells grown to different growth stages and at various temperatures is presented.