Small quantities (0.1, 0.5 and 1.0 weight percent) of rare earth oxides (La2O3, CeO2, Sm2O3, Gd2O3) and Y2O3 were added to a square-loop Mg-Mn ferrite which contained some ZnO and CuO. The effect of these additions on the static hysteresis loop and the signal to noise ratio was investigated. The samples were prepared by the usual oxide method, sintered 2 hours in air at 1300℃ and quenched from l050℃ or 950℃. Preli-minary results showed that La2O3 or Gd2O3 increased the squareness ratio (B-Hm/2)/Bm and the signal to noise ratio (uV1)/(dV0). Furthermore, for a large number of cores, the distribution curve of (uV1)/(dV0) was shifted as a whole to higher values of (uV1)/(dV0). In contrast, CeO2, Sm2O3and Y2O3 additions did not show these effects. The saturation magnetization and the Curie point were not appreciably effected by the introduction of the rare earth oxides. The influence of the core size, the quenching temperature as well as the content of rare earth oxides on the squareness ratio and the signal to noise ratio was also noted.