We have started a survey of the molecular gas content of spiral galaxies in high and low density regions (HDS and CS), selected according to well-defined criteria (Maia et al. 1994, ApJS, 93, 425). The HDS sample is formed by galaxies that are in groups of three or more members. The groups are defined such that they have a density contrast δρ/ρ ≥ 500. This is equivalent to densities larger than 18 galaxies/Mpc3. The CS sample is made up of galaxies which are not members of any group and which are situated in a region with a density contrast δρ/ρ ≤ 0.01, i.e. less than 0.0004 galaxies/Mpc3.