Fructosamine-3-kinase (FN3K) mediates the regeneration of lysine from fructosamines formed on proteins as a result of the 'early' Maillard reaction. As fructosamines and advanced glycation endproducts derived therefrom are supposed to play an adverse role in the development of diabetic complications, FN3K is discussed as a protein-repairing enzyme. In this study, a method for the determination of FN3K activity in erythrocyte lysate is described which overcomes the complexity of currently known assays. The assay is based on the FN3K-dependent conversion of the synthetic UV-active fructosamine Nalpha-hippuryl-Nepsilon-(1-deoxy-D-fructosyl)lysine (BzGFruK) to Nalpha-hippuryl-Nepsilon-(phosphofructosyl)lysine (BzGpFruK). The FN3K activity was quantified by measuring the formation of BzGpFruK using RP-HPLC with UV detection. Identification of the metabolite BzGpFruK was achieved by means of UV and mass spectroscopy. The results are related to the content of haemoglobin for standardisation. First activity measurements with a chosen number of normoglycaemic subjects confirmed the convenient applicability of the method and showed distinctly different individual activities, as already discovered recently. The new established assay needs only the equipment of a routine laboratory with HPLC instrumentation. This should facilitate further studies about a possible relationship between the FN3K activity and the development of diabetic complications.