A simple, automated, enzymic assay for the analysis of D-isocitrate in urine is described. The method detects D-isocitrate in the range 0·;05–1·;00 mmol/L, producing within-batch and between-batch coefficients of variation of 1·;8–3·;2% and 2·;7–3·;0% respectively. The assay was also shown to be robust, tolerating extremes of sample pH. Of a wide range of substances tested, only Fe3+ ions interfered, but only at levels considerably in excess of those encountered in urine. D-isocitrate was stable in urine for up to one week at room temperature or at 4°C, with or without preservatives.