The goal of the present research was to investigate therapeutic and prophylactic effectiveness of levofloxacin following experimental challenge of chickens with varying doses of Escherichia сoli namely 1 minimum lethal dose (DLM) and 4 minimum lethal doses (DLM). We conducted two experiments on two-day-old Hisex Brown chickens. For the first experiment we used 6 groups of chicks (n = 25, in each group). The drug was administered via drinking water (with free access) 24 hours before the challenge. Chicks of Groups 1 to 4 received levofloxacin via drinking water at concentrations 50 mg/L, 100 mg/L, 200 mg/L and 300 mg/L for 5 days. Group 5 was control, Group 6 was intact. The chicks were challenged with 1.5 х 108 E. сoli bacterial cells per 0.5 mL (1 minimum lethal dose (DLM)) intraperitoneally. The second experiment was conducted similarly but the infecting dose was increased by 4 times (4 minimum lethal doses (DLM)).Administration of levofloxacin at concentration 300 mg/L both in the first experiment, when the infecting dose was 1 minimum lethal dose, and in the second experiment, when the infecting dose was 4 minimum lethal doses, proved that the drug has the same high therapeutic and prophylactic effectiveness of 96 %. Administration of the drug at concentration 200 mg/L showed 100 % effectiveness in the first experiment. But in the second experiment levofloxacin at the same concentration ensured survival of only 88 % of the challenged chicks. When the drug was administered at concentration 100 mg/L we observed reliable change in indices. Accordingly in the first experiment 88 % of the chicks survived. In the second experiment, when the infecting dose increased to 4 minimum lethal doses, only 52 % of the chicks survived. Administration of levofloxacin at concentration 50 mg/L did not show significant therapeutic effect.