A radioimmunoassay (RIA) for urinary 6-sulphatoxymelatonin (aMT6s) using an 125I-aMT6s tracer is described. Iodinated aMT6s used as the label was synthesised by direct iodination of aMT6s using 1,3,4,6 tetrachloro 3,6 diphenylglycouril as the oxidant. The assay shows low cross reactivity with related compounds. Serial dilutions of 1:250 and 1:125 diluted urine gave parallel displacement curves. Comparison of the new RIA using 125I-aMT6s with the RIA using 3H-aMT6s label gave good correlation, as did comparison with a gas chromatography mass spectroscopy (GCMS) for total free and conjugated 6-hydroxymelatonin.