DURING some work in this Research Department on compounds of uranium, as a safety precaution, we commenced to analyse the urine of personnel concerned, using a fluorimetric method. In the preparation of fluorimetric standards, known amounts of uranyl nitrate were added to samples of urine from persons not engaged on the work with uranium. To our surprise we found uranium to be present in some of the 'blank' urine samples. It was found that analysts who had recently been engaged in the determination of sodium as sodium uranyl magnesium acetate1 voided traces of uranium in their urine, the element being detected for some weeks after the analyst ceased to be using ‘sodium reagent’. Out of 14 analysts examined between January 28, 1944 and February 1, 1944, six (Nos. 1–6 below) who had been in contact with magnesium uranyl acetate solution during the past two months had urine containing 2–10 µgm. per litre of uranium, eight (Nos. 7–14) who had not been in such recent contact with uranium salts varied from ‘not detected’ to 4 µgm. per litre. The table gives the experimental figures, expressed as μgm. uranium per litre. The fluorescence was compared visually, against standards equivalent to 2.6 or 10 µgm. per litre, 2 µgm. per litre being the limit of detection by the method used.