The Mariner 5 ultraviolet measurements obtained while the spacecraft was in interplanetary flight are interpreted as Lyman-α radiation. This radiation may arise from (1) the scattering of solar Lyman-α radiation by interplanetary atomic hydrogen, (2) the scattering of solar Lyman-α by atomic hydrogen that is present at the edge of the solar system, or (3) diffusely scattered Lyman-α radiation from the Galaxy. The Mariner 5 measurements show a symmetry with respect to the galactic equator which suggests that the major source of the observed emission is the diffuse galactic radiation. In the Münch model for the diffuse Lyman-α radiation from an HI region, the sources of the Lyman-α photons are stellar chromospheres, the photons are scattered a large number of times by interstellar atomic hydrogen, and are absorbed by interstellar dust. Some of the Mariner 5 measurements may be attributed to Lyman-α emission from an HII region since the field of observation scanned the edge of the Gum nebula.