Oxalic acid is separated from interfering substances by extraction with tri-n-butyl phosphate followed by co-precipitation with calcium sulphate. The precipitated oxalic acid is then reduced to glyoxylic acid, which is coupled with resorcinol to form a coloured fluorescent complex. 2. The spectrofluorometric method described is sensitive and highly specific, the minimum detectable amount of oxalic acid being 0.9mumole under the recommended conditions. 3. The concentration of oxalic acid in blood from 15 normal adults was 200-320mug./100ml. For serum the range was 135-280mug./100ml. The urinary excretion of oxalic acid by 60 normal adults on a normal diet was 9.0-28.5mg./24hr.
