The object of this investigation was to determine, if possible, whether or not the magnesium ion concentration in the backfill has a significant effect upon the current efficiency of a pure magnesium anode when used to protect a steel cathode. Accordinging to H.A. Robinson, there should normally be no effect, but he indicated that there is room for doubt in some cases. It may be concluded, from the results obtained in this experimental work, that the current efficiency of magnesium anodes, when used as cathodic protectors of steel, is generally unaffected by changes in the concentration of magnesium sulfate in the backfill. It may be further stated that the presence of manganese ion also has no effect upon the current efficiency. The results indicate the possibility that current efficiency is appreciably affected by changes in the current density at the anode. It is suggested that further work be done in the study of this effect. It is further suggested that a wide variety of backfill conditions, for example, soils of different pH, be used in conjunction with this study. / M.S.