The article is devoted to the development and synthesis of a new drug for use in animal husbandry and veterinary medicine as an immunostimulatory and adaptogenic agent. The basis of the new drug is a combination of magnesium, phosphorus, and casein of cow's milk. Given the important scientific and practical importance for veterinary medicine of innovative drugs that would have pronounced immunostimulatory and adaptogenic properties, the aim of our research is to develop a method of obtaining a new drug with an original composition and further studies of its effectiveness and safety. Modern materials and methods were used to achieve this goal. In particular, mass spectrometry was used on a Waters H-class UPLC liquid high-pressure liquid chromatography spectrometer with a Waters TQ-S micro three-quadrupole detector; atomic emission spectrometer with inductively coupled plasma Analytik-Jena Plasma Quant PQ 9000 Elite; liquid chromatographer with a three-quadrupole mass detector and with analytical column – Waters ACQUITY UPLC BEH C18 1.7μm 2.1x50mm. The result of the work was the development of a method of modification of the casein molecule, which was carried out in several stages: the first stage was the direct phosphorylation of the casein molecule; the second stage of the synthesis was the chelation of magnesium with casein. Based on the results of this work, the following conclusions were formed: 1) the synthesis of a new drug is carried out in two stages: the first – modification of casein by direct phosphorylation and the second stage – chelation of magnesium with casein; 2) it was found that the efficiency of phosphorylation directly correlates with the number of treatment cycles of the reaction mixture and is optimal for three treatment cycles