The experiment was carried out on 30 multiparous Black-and-White Lowland cows from 45 to 70 days of lactation, divided into three groups according to calving date and milk yield, and fed a basic ration consisting of maize, grass and sugar beet pulp silages with concentrate for the control group. In both of experimental groups, 20% of concentrate was substituted with endosperm from milk thistle (Sylibum marianum L.) and 3% of calcium carbonate, but in one of the experimental groups, 1.5% of buffering sodium carbonate was additionally added to the concentrate mixture. Endosperm of milk thistle contained 23% fat with a high proportion of unsaturated fatty acids. The mean fat content in the milk of the experimental groups was similar (4.83 and 4.61%, respectively) but significantly higher (P < 0.05) compared with the control group (4.27%). A decline in the percentage of short-chain (C_8:0, C_10:0, C_12:0) and medium-chain (C_14:0, C_16:0) fatty acids was observed along with a simultaneous advantageous, from the dietary point of view, increase in the percentage of unsaturated fatty acids (C_18:0, Δ-6-cis-C_18:1, Δ-9-12-cis-C_18:2) in the milk of cows of the experimental groups.