1. A simple method for diagnosing adenine phosphoribosyltransferase (APRT) deficiency using urine is described. 2. T.l.c. of 1 microliter urine from a child with APRT deficiency was performed and adenine was easily detected by its brilliant blue phosphorescence at liquid nitrogen temperature. 3. Four physicochemical characteristics of adenine were recorded: RF value, and the colour, decay time and pH sensitivity of phosphorescence. 4. Adenine was not detected in the urine of 116 subjects used as controls. These included healthy individuals and patients with inherited metabolic disorders, diseases of purine metabolism and of the kidney and urinary tract. Some of them were taking a variety of drugs including purine derivatives. 5. The test correctly diagnosed three cases of APRT deficiency out of 10 urine samples tested blind.