Incubation of human erythrocyte with 50 mM glucose results in glycosylation of aldose reductase besides hemoglobin A and other proteins. The glycosylation of aldose reductase is established by adsorption of the enzyme on phenyl boronate (PBA-60) column. Furthermore, the enzyme was purified to an apparent homogeneity from the erythrocytes incubated with 50 mM glucose and the glycosylation was quantitated by reduction with tritiated sodium borohydride. The glycosylated aldose reductase exhibits lower Km for glucose and NADPH as compared to unglycosylated enzyme and is not inhibited by phosphorylated intermediates such as ADP, 1,3-DPG, 2,3-DPG and 3-PGA, whereas physiological concentrations of these intermediates almost completely inhibit the unglycosylated enzyme. In addition, the glycosylated enzyme is less susceptible to aldose reductase inhibitors such as sorbinil and alrestatin. In hyperglycemia, blood glucose higher than 11 mM, almost all the aldose reductase is glycosylated.