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Yaks are the large herbivorous ruminants that live in high mountain pastures at altitudes from 2,000 to 4,000 m above sea level. This environment is quite harsh, and yaks calmly tolerate extreme cold — down to minus 40 ºС. The main food of yaks are herbaceous plants, the vegetation season of which in a cold mountain climate is short, about 3–4 months, and therefore they feed mostly on wilted pasture, which is, in principle, sufficient for these animals. At the end of winter, when the amount of pasture feed decreases, the animals become malnourished and lose weight. Therefore, yak farmers often face the need to feed their animals in winter. A complete ration of an animal, along with other factors, has a significant impact on the composition of milk and its productive ability. The aim of the work is to study the chemical composition, in particular the protein fraction, of the milk of yaks living in the mountainous regions of Kyrgyzstan in comparison with the milk of yaks from different regions of the world. The object of the study was milk of yaks inhabiting the highland regions of Naryn province of Kyrgyzstan. Samples were collected and tested according to standard methods. Studies have shown that the milk of yaks from the Kyrgyz population contains more protein (5.66%) than milk from yaks in the mountains of China, Russia and Mongolia (4.95, 4.55 and 5.3%, respectively). Milk we studied is high in amino acids such as methionine, lysine, proline, phenylalanine, and alanine. Seasonal (spring, summer) changes in the protein composition (whey proteins, casein proteins), amino acid profile, total nitrogen, non-protein nitrogen of yak milk were also studied. The total content of amino acids in the milk of Kyrgyz female yaks is noticeably higher in spring (6189.96 mg/100 g) than in summer (5101.47 mg/100 g). The research results presented in this article showed that the milking season and the associated feeding ration of yaks affects the composition of milk, which is associated with the nutritional value of grass feed and its nutrient content. Spring milk contains more proteins and amino acids than summer milk, which is due to the high protein content of plants in the spring period before the flowering phase. Milk of Kyrgyz yak with proven high nutritional and biological value is an alternative to cow milk.