The objective of this study was to determine the influence of rumen-protected protein supplemented with the amino acids lysine (Lys), methionine (Met) and histidine (His) added either as a powder or in the form of rumen-protected tablets to the rumen of dairy cows on the yield, composition and technological suitability of milk. The experiment was carried out on three lactating Holstein cows with an 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 4 d experimental period). In the first period one cow received the tablets (T) and the other two received the non-tableted mixture (C) with the same composition. In the subsequent period the design was opposite. Cows were fed a diet based on maize silage, lucerne hay and a supplemental mixture. Powder or tablets consisted of purified soya-protein HP 300, Lys, Met and His. Average milk yield in C cows was 16.73 kg and was significantly lower than in T cows (17.8 kg; P<0.05). Protein, fat and lactose contents were unaffected by the treatment. Protein yield was lower (529.5 g/d) in the C than in the T (569.3 g/d; P<0.05) cows. The increase in milk protein yield in T cows was followed closely by progressive increases in casein content (2.46 vs 2.68%). The content of short- and medium-chain fatty acids was higher and the proportion of C18:1, C18:2, C18:3n3 and C20:1 was lower in the C than in the T (P<0.05) animals. The proportion of UFA (both MUFA and PUFA) was increased and that of SFA in milk (P<0.05) was decreased in the T animals. The total SFA:UFA ratio was higher in C compared with T cows (1.89 vs 1.52; P<0.05).