COS-B gamma-ray observations are compared with large-scale HI and CO surveys in different regions of the sky to derive information on the cosmic-ray density distribution throughout the Galaxy and/or a calibration of the ratio between H 2 column density and integrated CO line intensity [ N(H 2) W co ] . In the region of the large complex of interstellar clouds in Orion and Monoceros the observed gamma-ray emission can be explained in terms of interactions between cosmic rays distributed uniformly with the local density in this region, and the interstellar gas. In the outer galaxy a steep negative gradient of the emissivity for the 70–150 MeV energy range is found, interpreted as a steep gradient in the cosmic-ray electron density. The emissivity for the 300–5000 MeV range is found to be approximately constant (within 20%) and equal to the local value out to large (∼20 kpc) galacto-centric distances, interpreted as a near constancy of the nuclear component. Results sofar available for the inner galaxy indicate that this trend, a stronger gradient in the electron distribution compared to that of the nuclei, can be extrapolated into the Galaxy. Calibration of the ratio N(H 2) W co in the Orion, Monoceros region and in the first galactic quadrant give consistent values in the range (2.5–3) 10 20 molecules cm −2 K −1 km −1 s. The galactic centre region is found to be an anomaly: (i) the density of cosmic-ray nuclei with energies of ∼1 GeV is anomalously small relative to the local value, or (ii) perhaps these cosmic rays do not efficiently penetrate the molecular clouds in the central region of the Galaxy (contrary to the case of Orion) or (iii) the N(H 2) W co ratio is anomalously low.