Aim of the study. We attempted to obtain the Mn-HMPAO complex (Mangoxim) and to evaluate the possibility to apply as a paramagnetic contrast agent for enhancement of tumors at MRI. Materials and methods. The synthesis of HMPAO was carried out by using R. D. Neirinckx — J. Pijarowska-Kruszyna technique (2017), without separation of enantiomers. The Mn-HMPAO was obtained by mixing Manganese(II) carbonate with HMPAO, pH 6,2–6,5, 0,5 M. Also the quantum chemistry calculations were carried out on the structure of Mn-HMPAO using method of functional of density DFT rb3lyp/6–31+g(d,p) for isolated molecules and for water solutions (CPCM). MRI study of Mn-HMPAO uptake carried out in twelve laboratory mouses with transplanted Lewis sarcoma, in T1-w. SE mode, TR=500 ms, TE=15 ms, 256×256 matrix, FOV 15×15 cm, 3 mm thin slice. MRI acquired in 15–17 min and 60–90 min after intravenous injection of the Mn-HMPAO. The uptake was quantified as index of enhancement: IE=(Int. T1-w.MRI)Mn-HMPAO/(Int. T1- w.MRI)pre-contrast. Results. The N,N-Mn-dioxim is more stable than O,O-Mn-dioxim for 50,5 KCal/mol, so in the nature only N,N-Mn-dioxim exists. R1 of Mn-HMPAO was 3,35 1/(mM•s), below R1 of gadopentetate=3,96 1/(mM•s). The highest value of early IE was over the tumor (3,36±0,31) and over lungs (3,07±0,43). At 60 min the IE over tumor decreased to 1,58±0,14, remaining far over IE values over any other location. Conclusion. The Mn-HMPAO complex demonstrates paramagnetic properties for the application as the contrast MRI agent and probably will improve the selective enhancement of tumors.