Herein we present a study on effectiveness of Levofloxacin in experimental colibacillosis of laboratory animals and chickens. The experiments were conducted in vivarium settings. In the course of the experiments we evaluated effectiveness of Levofloxacin administered in drinking water at different concentrations for treating laboratory animals and chickens which had been experimentally infected with Escherichia coli. For the first experiment white mice were divided into 7 groups, 40 per group. We began treat ing the mice with the antimicrobial at concentrations 300 mg, 200 mg, 100 mg, 50 mg and 25 mg Levofloxacin per l in drinking water in every group respectively 24 hours before the challenge with Escherichia coli in suspension at a dose of 1.5 x 10^7 CFUs. Levofloxacin was administered for 5 days. Analysing the outcome of the first experiment we concluded that satisfactory result was only achieved in the group of mice which had been prescribed Levofloxacin in drinking water at a concentration of 300 mg/ L for 5 days. The survival rate for this group was 65%. For those groups receiving Levofloxacin in drinking water at concentrations of 200 mg/L, 100 mg/L, 50 mg/L and 25 mg/L the desired effect was not achieved. For the second experiment we divided day-old Hisex Brown chicks into 7 groups (25 chickens per group). They began receiving Levofloxacin one day before the challenge with Escherichia coli in suspension at a dose of 3 x 10^8 CFUs. Group 1 received Levofloxacin in drinking water at a concentration of 25 mg/L for 5 days, Group 2 – 50 mg/L, Group 3 – 100 mg/L, Group 4 – 200 mg/L and Group 5 – 300 mg/L. Our research demonstrated that Levofloxacin, when administered in drinking water to challenged chickens at concentrations of 100 mg/L, 200 mg/L and 300 mg/L, has high therapeutic effectiveness (96%). Groups that received Levofloxacin at concentrations of 25 mg/L and 50 mg/L showed worse results. Thus, the survival rates were 64% and 80% respectively. The mortality rate in the control group was 84%.