The aim of this study was to determine the effect of soya-protein enriched with amino acids: lysine (Lys), methionine (Met) and histidine (His) added either as a powder or in a form of rumen-protected tablets into rumen of dairy cows on the percentage and yield of casein, changes in the proportion of casein fractions resulting in variations in amino acid profile of milk, casein and non-casein protein. The experiment was carried out on three lactating Holstein cows of average weight of 523 kg fitted with ruminal and duodenal cannulas. The experiment was divided into 4 periods of 14 d (10 d preliminary period and a 4 d experimental period). In the first period one cow received the tablets (T group) and the other two received the non-tableted mixture (C group, control) of the same composition. In the subsequent period the rate of animals was antipodal. Cows were fed on diet based on a maize silage, lucerne hay and a supplemental mixture. Tablets or mixture consisted of purified soya-protein HP 300, Lys, Met and His.The casein content and yield was higher in the group T in comparison to the group C (2.68% and 476.55 g vs 2.46% and 408.43 g; P<0.05). Content of β- and κ-casein was unaffected by the treatment while content of α-casein was lower in the T group (54.10 vs 55.98%; P<0.05). Yield of every casein fraction was significantly higher (P<0.05) in the T group than in the C. The increases in the casein yield resulted in significantly higher(P<0.05) yields of individual amino acids in milk and casein in the T group compared to the control. In the experiment we found out that the concentration of Thr in milk and Thr, Pro and Met in casein was significantly different(P<0.05) in the T group compared to the C. Duodenal flows of individual amino acids through the duodenum were determined.