Nanomedicine is a great interesting area for use of magnetic nanoparticles. Among their main applications we can highlight, for example, its use as contrast agents for magnetic resonance imaging, or intracellular delivery, or cellular hyperthermia, which is an employed method in oncology therapy. This study aimed to determine whether the magnetic nanoparticles with La_0.78Sr_0.22MnO₃ (LSMO), coated with SiO₂ are cytotoxic to the cancer cell line MCF-7 (human breast adenocarcinoma). For perovskite LSMO synthesis the liquid-mix method was used and sol-gel method was used for SiO₂ coating. Cytotoxicity was determined by cell number and viability assessment as well as clonogenic assay to study chronic effect induced by nanoparticles. Concentrations of 74.88 ± 1.1 LSMO µg/ml were able to reduce 50% of cell number compared with control untreated cells and after 72 h of treatment, cells lead a slight chronic effect. We also observed morphological changes after incubation of particles.