A single dose of danofloxacin (1.25 mg/kg b.w.) or marbofloxacin        (2 mg/kg b.w.) were injected intramuscularly to ten dairy cows between 3.5 and 6.5 years old and weighed 500 to 580 kg. for evaluation its plasma and milk concentrations as wellas its antibacterial characteristics against comman mastitispathogens. Blood and milksamples were collected for determination of concentrationsof two drugs by HPLC   method. Results showed that, the passage of two drugs from blood into milk were rapid and extensive specially danofloxacin which at 4 and      8 hrs. its milk concentration was 2.4 and 4.2 times higher than the plasma concentration, but this ratios with marbofloxacin was 1.15 and 1.14 at the same time respectively. Danofloxacin concentrations were maintained in the cow's milk equal to or higher than the MIC₉₀ of E.coli (0.06 mg/ml) and Mycoplasma spp. (0.008-0.5 mg/ml) for 24 hrs. and of S. aureus (0.18 mg/ml) for 12 hrs.. The marbofloxacin concentrations were still equal to or higher than MIC₉₀ of E.coli (0.016 mg/ml) for       24 hrs. and of S. aureus (0.229 mg/ml) and Mycoplasma spp < em>.            (0.48 mg/ml)for nearly 8 hrs.