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We propose to carry out broad-band imaging with ISOCAM of a sample of nearby elliptical galaxies for which we have already obtained an accurate mapping of the dust absorption and of the line-emitting (Halpha+NII), warm gas component from optical data, as a part of our now completed ESO Key Programme "Towards a Physical Classification of Early-Type Galaxies". It is now clear that as much as 80% of the ellipticals contain dust, while about 53% show warm ionized gas. Puzzingly though, the amount of dust mass inferred from optical observations is usually small, an order of magnitude less than the estimate from IRAS. Also, dust and gas are usually associated and do not follow the general stellar distribution, strongly suggesting an external origin for them. To tackle the important problem of the origin and evolution of the ISM in ellipticals on a quantitative basis, we propose to image at mid-IR wavelengths through the LW1, LW3 and LW7 filters to map the distribution of the warm dust component (including the silicate band) and to derive its physical properties, such as temperature and mass. By correlating these properties with the other characteristics of the host galaxies, such as the radio and X-ray luminosities, optical dust lanes, Halpha emission and distribution, we will directly test the hypothesis that dust and gas have an external origin and that a fraction of the warm component, responsible for the mid-IR diffused emission, originates in the winds of evolved stars. This will also provide crucial information about the relevant physical processes taking place in the ISM. For instance, it has been suggested that the X-ray coronae of elliptical galaxies provide a very hostile environment for dust and so by mapping the intensity of dust emission we will be able to probe whether there is dust formation and destruction at work. The high quality of our dust absorption and emission-line images coupled with the excellent resolution capabilities in the mid-IR of ISOCAM will enable us to investigate the relationship between the optical colors (V-R) and the presence of dust, its association with the optical dust lanes and its co-spatiality with the gas. ISO is the only facility offering mid-IR imaging capabilities with a resolution high enough to allow meaningful comparisons with optical images.