Galactose-1-phosphate uridyltransferase (Gal-1-PUT or GALT) quantitation is important in the diagnosis of Classical Galactosaemia. An HPLC-MS/MS assay has been set up, optimised and validated in our lab at North Bristol NHS Trust. Benefits include no radioactivity, smaller sample volume, larger batch sizes and improved precision. A summary of the method, its performance characteristics, and comparison with the previous assay is presented, alongside sample requirements and clinical indications.