In the experiment involved 186 milk samples obtained from cow udder quarters at different state of mastitis. Milk derived from infected udder quarters was characterised by a statistically significant higher total protein percentage share and a higher pH value (p£ 0.05) by a lower lactose (p£ 0.01) and casein content (p£ 0.05) than milk of healthy quarters. The content of solids not fat indicated the upward tendency and the content of total solids and vitamin C was reduced. The clinical mastitis caused reduction of milk yield by 33.63% and its components by 24.11 to 52.20%. Mastitis was most often induced by the strains of Staphylococcus aureus. Different cows reaction to mastitis was observed.