The common antibiotics used for the treatment of mastitis are Benzylpenicillin, Gentamycin, Tetracycline and Oxytetracycline. The antibiotic residues are being secreted to milk if the animal milk is drawn within 72 hr. These antibiotic residues in milk inhibit the growth of lactic acid bacteria used for fermented milk products as they inhibit on the cell wall synthesis and protein synthesis. The most sensitive organism for these antibiotics is Bacillus subtilis, Bacillus stearothermophilus and Streptococcus thermophilus. In this experiment the antibiotics are being tested against the indicator bacteria Bacillus subtilis by using the disc assay method and inhibitory zone is determined at the lowest concentration level. The benzylpenicillin produced inhibitory zone of 15 sq mm at the concentration of 0.24IU/ml, Gentamycin produced 20 and 14.5 sq mm at the concentrations of 0.4 and 0.04 µg/ml, oxytetracycline produced 30 and 28 sq mm at the concentrations of 50 and 5µg/ml. S.thermophilus when used as indicator organism was showed that the S.thermophilus was not inhibited at concentrations of 0.024 and 0.0024IU/ml of benzylpenicillin, gentamycin inhibited the organism at the concentrations of 0.4 and 0.04 µg /ml oxytetracycline inhibited the organism at the concentrations of 50 and 5 µg/ml respectively.