A modification of the resorcinol method of Kulka(1) for the determination of ketoses is described. Though being a stopped enzyme test, it is much more sensitive than the carbazol method and by applying microtiter plates and measuring with an ELISA reader, a large number of tests can be performed within a short time, thereby facilitating initial velocity studies. The test is linear up to a concentration of 2.5 mM D-xylulose even in the presence of to mM D-xylose and 2 mM D-fructose in the presence of 10 mM D-glucose. The sensitivity is 25 mu M for xylulose and 38 mu M for fructose. The test method is insensitive to perturbations of substances frequently used in isolation procedures such as ammonium sulfate, Triton X-100, PEG 6000, sodium dodecyl sulfate, and ethanol in moderate concentrations.