Digital Holographic Microscopy (DHM) has been used to investigate the spontaneous cell membrane fluctuations (CMF) of the Red Blood Cell. DHM as an interferometric technique is able to accurately provide the wavefront deformation induced by a transparent specimen, including living cells in a transmission configuration. From a numerical reconstruction of a single hologram, quantitative phase contrast images are obtained. The local phase shift is proportional to the specimen thickness with accuracy of 5-10 nm. As a non invasive full field technique DHM is particularly well suited to assess and study membrane fluctuations of a large number of cells simultaneously. In our analysis we show that CMF amplitudes are unhomogenously distributed on the cellular surface and seem to correlate with the biconcave equilibrium shape of erythrocytes. A mean fluctuation amplitude of 47 nm is measured in a group of 198 erythrocytes.