Aureomycin Mastitis Suspension was given intramammary to 44 cows (Lowland black-and-white breed) with sub-clinical mastitis. The treatment involved 76 lobes of the mammary glands. The results were compared with those obtained in the therapy of 38 lobes of the udder (22 cows) treated with Syntarpen 500 mg Polfa. A single dose of Aureomycin Mastitis Suspension contained 426 mg of chlortetracycline hydrochloride and 2 mg of hydrocortisone. The other drug contained 500 mg of sodium cloxacillin. Streptococcus spp. (mainly Str. agalactiae), staphylococci coagulase-positive and Staphylococcus epidermidis were isolated from milk. All strains were sensitive to tetracyclines, 8 strains of Streptococcus spp. and Staphylococcus spp. were slightly sensitive to cloxacillin. The drug was administered 3 times at 24 h intervals. Responsiveness to the drug was assessed 14 and 28 days after the therapy had been stopped. Chlortetracycline with hydrocortisone resulted in 88,2% bacterial and 75,7% clinical recovery. Cloxacillin cured 57,9% and 47,4% of udder lobes, respectively. The difference in favour of Aureomycin Mastitis Susp. was significant (p ≤ 0,001).