Modulation sensitivity was measured as a function of the relative phase of two equiluminous chromatic sources (564 and 625 nm) for temporal frequencies ranging from 1 to 40 Hz. Measurements were made at a retinal illuminance of 900 Td, with a 2 degree field metameric to approximately 600 nm. For most frequencies, the phase of least sensitivity was other than 180 degrees, indicating the existence of a luminance response to antiphase modulation. Additional data sets examined the effects of field size (0.5 degrees-8 degrees) and mean chromaticity (583-610 nm). Variation in field size, chromaticity, and photometric setting had little effect on the phase of least sensitivity. The data were fitted by using vector summation between luminance and chromatic responses; the modulation-sensitivity functions that resulted from this analysis agreed with those of earlier studies.