A modular digital holographic microscopy system for integration in commercial microscopes has been developed. The reconstruction of the digitally captured holograms is performed by application of a spatial phase shifting non diffractive reconstruction method. As a consequence of the applied algorithms the reconstructed holographic images contain not the disturbing terms "twin image" and "zero order". In combination with microscope lenses the system's lateral resolution is improved up to the diffraction limit. Digital holographic focus adjustment with constant imaging scale allows multi focus imaging of object planes and subsequent focus correction from only a single captured hologram. Results of investigations on technical specimen characterize the lateral and axial resolution of the system. The applicability of the system is demonstrated by results obtained from drug stimulated cellular morphology changes and apoptosis monitoring of living pancreas cells.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.