Task of the HELIOS mission was the exploration of the inner heliosphere, inside the Earth's orbit to 0.3 AU. The Rome-GSFC magnetic field experiment has been a joint venture of an italian group (University of Rome and CNR/Istituto Fisica Spazio Interplanetario, Frascati) and an american group of NASA/Goddard Space Fligth Center, under the responsibility of F. Mariani and N.F.Ness, respectively. A description of the experiment can be found in the NASA-GSFC report X-692-75-112 (1975), by C. Scearce et al. The instrument is a dual configuration of two tri-axial fluxgate magnetometers operating at four different sensitivities (from 0.84 to 0.03 nT). The time resolution, depending the operation mode, telemetry format, and bit rate, for most cases varied from 0.07 to 1.5 seconds. This data set contains hour-averaged magnetic field vectors from the the Ness/Mariani experiment on Helios 1 [or 2]. Each record contains time, magnetic field intensity and cartesian components in spacecraft-centered solar ecliptic coordinates, standard deviations, numbers of fine-scale points in the average, and an instrument mode indicator. It should be noted that the "average" mode, which was used a minority of the time and involved on-board averaging, may have incorrectly handled range changes caused by occasional significant changes in ambient field intensity. The data set had not been filtered for this effect upon delivery to NSSDC. The data files are named either HE1MGA.asc, for one file covering the entire 1974 - 1981 period, or HE1MGAyy.asc, for one year intervals of data.