This study is conducted to evaluate the protective effectiveness live-attenuated E.Coli vaccine. A total of 140 broiler chicks were classified into seven groups; the first two groups (1 & 2) were labeled as challenge control for E.Coli O78 and O91 strains, the second two groups (4 & 5) were vaccinated with the E.Coli live-attenuated vaccine at day 1, the third two groups (3 & 6) treated with Difloxacin "second generation fluoro-quinolone" 1ml/L drinking water for first three days of life then vaccinated at day 7 with the E.Coli liveattenuated vaccine. The last group (7) was kept as a control negative group. The challenge was directed to the 21-day-old chicks with either homologous O78 or heterologous O91 E.Coli strains 1.2x10^9 Colony Forming Unit/mL injected intratracheally using 0.5 ml/ bird of each avian pathogenic E.Coli (APEC) strain. Clinical signs, gross and histo-pathological lesions, the intestinal total count of E.Coli (CFU), the chick body weight and the weight of lymphoid organs were calculated at 28 days of age. The results postulated that Poulvac® E.Coli vaccine gives a significant protective effect against the challenge with homologous O78 and the most common heterologous serotype O91. Based on the formentioned parameters, vaccinated birds at 7 days of age (Groups 3&6) which were treated with Difloxacin and challenged with both homologous and heterologous E.Coli serotypes (O78 & O91) showed superior protection over those vaccinated at day one of age (Groups 4&5), challenged with O78 & O91 E.Coli serotypes and did not receive any antibiotic.