Glioblastoma Multiforme (GBM) is an aggressive brain cancer. Immunotherapy is a promising avenue of exploration for treatment of GBM. We discovered a new drug capable of stimulating innate immunity and showed that it is able of reducing tumor growth in a mouse model of GBM. We used ultrasmall iron oxide nanoparticles (USPIO) to track the migration of stimulated immune cells and showed that this drug related response can be detected with as little as 7 days of treatment. MRI tracking of immune cells using USPIO is a promising tool that can be easily translated into the clinical setting.