A novel enzyme assay for screening glycosidases has been developed by using glycosyl fluorides. The method is based on the color change caused by the complex formation of fluoride ion and lanthanum-alizarin complexon (La3+-ALC). The assay showed higher sensitivity compared with the conventional method using p-nitrophenyl glycoside as screening substrate. A quantitative study on the hydrolysis of -lactosyl fluoride catalyzed by a purified cellulase has been demonstrated, indicating that the present method can also be employed for enzyme kinetics.