Environmental effects may be of great importance to the structure and evolution of galaxies. Sullivan and collaborators (1981, AJ 86, 919) find that the ratio between HI content and luminosity of galaxies tends to be smaller in regions of high galaxy density than elsewhere. It is not quite clear what processes reduce the gas content (or enhance the luminosity!): collisions, mergers or tidal interactions of galaxies, encounters with gas clouds, stripping by ram pressure? Detailed comparison of gas distributions and motions in cluster and field galaxies may help answer this question.