SUMMARYA simple and specific method for the assay of transketolase activity in erythrocyte hemolysates based on the measurement of NADH consumption is described. The test conditions are discussed and the results are compared with a calorimetric method. Normal values are given for the transketolase activity and the thiamine pyrophosphate effect.Most parenchymal organs contain transketolase (TK) (E.C. 2.2.1.1). The enzyme activity in these organs and in the red blood cells decreases readily, when a deficiency of thiamine developsI. Brin and associates2-4 have demonstrated, that the erythrocyte TK activity and specially the relative enhancement of TK activity in erythrocyte homogenates after saturation with the coenzyme thiamine pyrophosphate (TPP effect) in vitro is a sensitive index for the detection and evaluation of thiamine deficiency. Thiamine depletion experiments on human test subjectss-9 and on test anirnals5~6~10~11 have shown, that an increase of the TPP effect can be observed even before clinical symptoms are perceptible. After a prolonged deficiency also the saturated enzyme activity in vitro is decreasedc.The value of these assays is restricted somewhat by the observation, that patients suffering from hepatic cirrhosis-both withI and without13 neuropathies caused by vitamine B, deficiency-showed a diminished erythrocyte TK activity in combination with a normal TPP effect. Besides, the improvement of the liver function and the erythrocyte TK activity were found to be related. In patients with thiamine phosphorylating defect TK activities remained deficient (TPP effect increased) after thiamine treatment14. Erythrocyte TK activities exceeding the normal values with TPP effects within the normal ranges were reported in patients with Addisonian pernicious anemia15.TK catalyses the following metabolic steps in the metabolism of pentoses16:(I) xylulose+P+ribose+P --f sedoheptulose-7-P+glyceraldehyde-3-P (2) xylulose-5-P+erythrose-4-P --?' fructose-6-P+glyceraldehyde-3-P