An autoanalytical method for determination by the thiochrome fluorometry of thiamine (B_1), thiamine pyrophosphate (TPP) and thiamine disulfide (TDS) in blood (whole blood and plasma) or urine, has been studied using autoanalyser (Technicon Co.), and the following results were obtained. 1. The autoanalysis was accomplished almost satisfactorily by a) measurement of thiochrome fluorescence in aqueous state, b) reduction of TDS into B_1 with cysteine, c) isolation of TPP with ion-exchange resin, d) dialysis of the thiamines on the flow diagram of the analyser, and e) elimination of the most part of fluorescent substance contaminated in urine with ion-exchange resin. 2. Comparing with Permutit method (the conventional method), this method gives measured values with a slightly additive bias for blood, and with a rather negative bias for urine. However, the biases are small and almost constant for samples loaded or not loaded with the thiamines. Accordingly, the method can be applied to determine accurately increased or decreased concentrations of the thiamines in blood or urine. 3. In conclusion, since the method gives fairly good approximation of data to the conventional method, and enables to treat rapidly a lot of samples, it is a valuable method for some purposes, such as tracing thiamine levels in blood and excreted thiamines in urine after the administration of thiamine drugs, some clinical tests, and so on.