Neoculin, a sweet protein occurring in Curculigo latifolia, is unique in that it also has taste-modifying activity capable of converting sourness to sweetness. Calcium imaging analysis with HEK cells expressing the human sweet taste receptor, hT1R2/T1R3 demonstrated that the intracellular calcium concentration increased following the addition of 20 μM neoculin. The use of lactisole, a blocker of hT1R3, inhibited the intracellular calcium concentration increase almost completely. In sensory tests, when acetate buffers with different pH values were placed on the tongue after tasting neoculin, a higher intensity of sweetness was detected at lower pH. The sweetness was also suppressed with the addition of lactisole. These results suggest that both the sweetness and the taste-modifying activity are mediated via the human sweet taste receptor.