Harmonic phase-dispersion microscopy (PDM) is a new imaging technique in which contrast is provided by differences in refractive index at two harmonically related wavelengths. We report a new configuration of the harmonic phase-dispersion microscope in a Mach-Zehnder geometry as an instrument for imaging biological samples. Several improvements on the earlier design are demonstrated, including a single-pass configuration and acousto-optic modulators for generating the heterodyne signals without mechanical arm scanning. We demonstrate quantitative phase-dispersion images of test structures and biological samples.