Curculin elicited a sweet taste. After the sweetness of curculin diminished, application of deionized water or an acid to the tongue induced a sweet taste. The maximum sweetness of curculin itself was equivalent to the sweetness of 0.35 M sucrose. The maximum sweetness induced by 0.02 M citric acid or deionized water after curculin dissolved in a buffer of pH 6.0 was held in mouth for 3 min was also equivalent to that of 0.35 M sucrose. The sweetness induced by deionized water was completely suppressed by the presence of 1 mM CaCl2 or MgCl2, while that induced by an acid was not suppressed by the presence of divalent cations. Based on these results, the mechanism of the taste-modifying activity was discussed. Stability of curculin was examined under various conditions. The taste-modifying activity of curculin was unchanged when curculin was incubated at 50 degrees C for 1 h between pH 3 and 11.