Stoichiometric Mn0.2Zn0.8Fe2O4 monodisperse nanoparticles were prepared by the so-called polyol method. The variation of magnetization as a function of magnetic field H (up to ±50 kOe) and temperature (5−320 K) were investigated, for zero-field-cooled (ZFC) and field-cooled (FC) conditions on freshly produced powder. The T variation of the low-field (H = 200 Oe) magnetic susceptibility is characteristic of superparamagnets with a blocking temperature below room temperature. The H variation of the low temperature (T = 5 K) magnetization exhibits a hysteresis loop. The coercivity is weak, about 0.2−0.3 kOe, which is typical of soft-ferrimagnetic materials. The 0 K saturation magnetization and the Curie temperature are found to be 98 emu.g−1 and 360 K, respectively. The magnetic properties of the particles are discussed in relation with their chemical composition and their cation distribution in the opportunity of using them as heating mediators for hyperthermia application in cancer therapy. In this aim, the toxicity of the particles was also evaluated by viability assays on human umbilical vein endothelial cells (HUVEC).