An automated enzymic method is described for the determination of microgram amounts of oxalic acid. The acid is decarboxylated by a specific enzyme, oxalate decarboxylase (E.C.4.1.1.2), and the carbon dioxide evolved is measured colorimetrically in a modified Technicon AutoAnalyzer. The minimum detectable concentration of oxalic acid was about 5 µg per 100 ml. For serum it was necessary to remove protein first by ultrafiltration as it interfered with the recorder base-line readings. Phosphate and sulphate ions inhibited enzymic activity, which was allowed for by adding appropriate concentrations of these ions to the standard oxalate solutions used for calibration. The concentration of oxalic acid in normal human serum ultrafiltrates ranged from 80 to 140 µg per 100 ml (mean 118 µg per 100 ml). Similar values were observed for horse serum. The recovery of microgram amounts of oxalic acid added to serum averaged 110 per cent. The coefficient of variation of replicated determinations of oxalic acid was ±2 per cent. for aqueous solutions and ±5 per cent. for serum ultrafiltrates.