The phenomenon of spin-crossover (SCO) is one of the best examples of molecular bistability that means that in some transition metal complexes the spin state of the complexes changes due to external perturbation such as temperature, pressure, light illumination. The bistable compounds could be useful for the construction of memory and sensory molecular-based devices. The most often this phenomenon is found for Fe(II) compounds, it is rarely observed for Mn(III) compounds. Recently the spin-crossover phenomenon was revealed for the Mn(III) compound with the chemical formula [MnL2]NO3 , where L2 notes the methylene groups. [1] Here we present a microscopic approach to the SCO phenomenon in crystals containing manganese (III) ions as structural units and reveal the main mechanisms governing this effect. The model includes the interaction of the manganese ions with the crystal field of the surrounding ligands and the cooperative electron-deformational interaction. The model also takes into account that the crystal field of C2v symmetry splits the cubic 1 3T term of the MnIII ion into an orbital singlet and an orbital doublet . In this field the E 5 term is split into two singlets . The range of the crystal field parameters for which the low-lying part of the energy spectrum of the MnIII ion consists of two non-degenerate levels arising from the states 1 3T and E 5 with the spin S=1 state being the ground one is determined. It is shown that in this case the ground level mainly originates from the ( ) 4 1 2 3 T t term i.e. the situation facilitating SCO takes place. The ls-hs transition is considered as a cooperative phenomenon driven by the interaction of the electronic shells of the MnIII ions with the all-round full symmetric deformation that is extended over the crystal lattice via the acoustic phonon field. The SCO phenomenon is treated within the molecular field approximation. Quite good agreement has been obtained between the calculated and experimental values of the magnetic susceptibility and the effective magnetic moment.