The effect was studied of cow udder infection with S. aureus on changes of milk proteins. The tested milk was derived from two cows of the black-white lowland breed aged 5 years in the second stage of lactation. Proteins soluble and insoluble at pH 4.6 were characterized by means of starch-urea and polyacrylamide gel electrophoresis. The solution of nitrogen compound reacting with ninhydrin and soluble at pH 4.6 was separated by gel filtration, thin-layer chromatography and high-voltage paper electrophoresis, thin-layer chromatography and high-voltage paper electrophoresis. In the milk of the infected cow degradation of alpha s, beta and kappa casein, and increased content of gamma immunoglobulin. In whey proteins a rise was observed of serum albumin content and a decrease of beta-lactoglobulin and alpha lactoalbumin. In the solution after precipitation of casein at pH 4.6 in the milk of the infected cow a greater number of fractions reacting with ninhydrin was found. The observed range of degradation changes in milk protein in the infected cow (S. aureus) suggests that it has a negative effect on the technological, nutritional and hygienic value of milk.