Protamine was found to stabilize ascorbic acid (AsA) in aqueous solution. The concentration of AsA (50 μmol/l) dissolved in 0.05 M phosphate buffer, pH 7, decreased by 57% after standing for 90 min at 30°C, whereas 91% of AsA remained under the same conditions in the presence of 60 μmol/l protamine. When 0.5 μmol/l of copper ion (II) was added to the above AsA solution (0.05 M phosphate buffer, pH 7) in the absence of protamine, AsA decreased by 5% after standing for 15 min at 30°C. On the other hand, 91% of AsA remained after 15 min and 52% of AsA remained after 90 min under the same conditions in the presence of 60 μmol/l protamine. Furthermore, the effect of protamine on the stability of AsA in various pH solutions (pH 1-10) was examined after standing for 90 min at 30°C. Degradation of AsA was accelerated by increasing the pH from 1 to 10. On the other hand, the effect of protamine (60 μmol/l) on the stabilization of AsA was recognizable above pH 5 and degradation of AsA was controlled at higher pH. When 0.5 μmol/l of copper ion (II) was added to the above solutions in the absence of protamine, degradation of AsA was greatly accelerated above pH 5. When 60 μmol/l of protamine was added to the solutions, the effect of protamine on the stabilization of AsA was observed above pH 5 and degradation of AsA was controlled at higher pH.