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Determination of acetaldehyde in human blood by the semicarbazide method has been studied. An artefactual production of acetaldehyde from ethanol in blood and plasma of about 20 mumol/l was observed at concentrations of ethanol of about 60 mmol/l. This artefact was reduced to less than 1 mumol/l after addition of thiourea. The presumably non-enzymatic production of acetaldehyde from ethanol was demonstrated by the release of 3H from (1R)-[3H]ethanol added to the blood immediately after sampling. The results demonstrate that oxidation of ethanol is the major cause of the artefactual acetaldehyde formation. In human volunteers, metabolising ethanol, very low concentrations of acetaldehyde were found by the modified method, which includes thiourea.