Electron paramagnetic resonance (EPR) and static magnetic susceptibility measurements were carried out on lanthanum-manganese substituted (up to 10 mol%) strontium titanate polycrystalline samples. X-ray diffraction (XRD) analysis revealed in each sample a single cubic perovskite phase with a different lattice parameter. Only weak and narrow EPR signals from a small fraction of Mn 2+ and Mn 4+ ions were observed when the La and Mn content did not exceed 5 mol%, indicating the nearly complete stabilisation of the Mn 3+ oxidation state. Heavily substituted samples showed another EPR signal, very broad and with intensity and resonance field strongly temperature dependent. For these samples, a complex dynamics of Mn 3+ stabilisation is suggested, supported by micro-Raman data.