We present two digital image processing methods for image quality enhancement in Digital Holographic Microscopy (DHM). With the first method, the Fourier transform of an off-axis hologram is spatially filtered in order to eliminate the zero order of diffraction and the twin image. We show that this operation enhances the image contrast and the signal to background ratio. The second method describes an original technique to perform the apodization of the hologram with a transmission profile which follows a cubic spline interpolation. The shape of the apodized aperture can be optimized by adjusting iteratively four parameters which define entirely the transmission of the aperture. We show that this operation reduces artifacts produced by diffraction effects associated to numerical reconstruction process. (C) 1999 Optical Society of America.