The perceived whiteness of a white object was investigated using psychological experiments. Six types of whitish light sources were used; their spectral distributions were varied by using four types of colored filters. The illuminance level was kept constant at 1000 lx. Paired comparison was used to construct the interval scale for the perceived whiteness. A new index was developed to evaluate the perceived whiteness under various illuminants. The index is proportional to the ratio between the chromatic and achromatic responses in the visual system. The chromatic response is based on opponent color theory. The index was highly correlated with the results of the experiments. It can thus be used to evaluate perceived whiteness under various illuminants and suggests that the chromatic response affects the perceived whiteness.