We present the results of an optical imaging survey of 73 luminous early-type galaxies directed to study the properties of their interstellar medium. We have found that a large fraction of E (72%) and S0 (85%) galaxies in our sample contain a measurable amount of ionized gas. The gas morphology appears to be rather smooth; however 12% of the sample galaxies show a complex and extended (as much as 10 kpc from the nucleus) filamentary structure. We find a significant correlation between Hα +[NII] and X-ray luminosities; the distribution of datapoints and upper limits suggests that the presence of warm gas is always associated with the presence of a hot gaseous halo, while there exist strong X-ray emitters with no Hα +[NII] emission. Similar, although less clear, characteristics are displayed by the L_{Hα +[NII]}-L_{IR 12microns } correlation. There is also a strong correlation between the blue-luminosity within the line-emitting region and the Hα +[NII] luminosity. This correlation indicates that the heating and ionization of the gas by ultraviolet photons from post-AGB stars is a very plausible and attractive mechanism to explain the observed Hα +[NII] luminosity.