Intact human erythrocytes catalyse the conversion of fructose into fructose 3-phosphate with an apparent K(m) of 30 mM [Petersen, Kappler, Szwergold and Brown (1992) Biochem. J. 284, 363-366]. The physiological significance of this process is still unknown. In the present study we report that the formation of fructose 3-phosphate from 50 mM fructose in intact erythrocytes is inhibited by 1-deoxy-1-morpholinofructose (DMF), a synthetic fructosamine, with an apparent K(i) of 100 microM. (31)P NMR analysis of cell extracts incubated with DMF indicated the presence of an additional phosphorylated compound, which was partially purified and shown to be DMF 3-phosphate by tandem MS. Radiolabelled DMF was phosphorylated by intact erythrocytes with an apparent K(m) ( approximately 100 microM) approx. 300-fold lower than the value reported for fructose phosphorylation on its third carbon. These results indicate that the physiological function of the enzyme that is able to convert fructose into fructose 3-phosphate in intact erythrocytes is probably to phosphorylate fructosamines. This suggests that fructosamines, which are produced non-enzymically from glucose and amino compounds, may be metabolized in human erythrocytes.