From 92 samples of quarter milk from cows with a clinical mastitis or showing an enlarged number of somatic cells, 10 different microorganisms were isolated. They occurred alone (48% of samples) or as a mixed flora. Streptococcus agalactiae (84.8% of samples) and Staphylococcus aureus (51.0%) belonged to the most frequently identified patogenes. The isolated microorganisms where insensitive or poorly sensitive to 11 out of the 15 tested antibiotics, whereas only 2 of them could be efficient in mastitis therapy. It was found that in order to improve the efficency of mastitis therapy, before its beginning it is necessary to prepare antibiograms defining the resistance to drugs of separate microorganisms species.