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The present study was conducted to evaluate dose-dependant immune response of Escherichia coli isolate in rabbits. Escherichia coli was isolated from mastitic buffaloes and characterized on the basis of morphological, cultural and biochemical tests. A total of 12 adult healthy rabbits, randomly divided into four equal groups (A, B, C and D) were used to evaluate the dose-dependent immune response to Escherichia coli antigen. Inocula containing 106 cells/ml of Escherichia coli were injected subcutaneously with the increasing doses of 0.20, 0.40 and 0.80 ml in groups A, B and C, respectively. The rabbits of group D were kept as un-inoculated control. Blood samples were collected at weekly intervals for four consecutive weeks from rabbits of all groups and analyzed for serum antibody titers. An increased geomean antibody titer (GMT), reaching 78.8, was observed in rabbits of Group C. The other two groups (Groups A and B) showed progressive increase in GMT up to day 21 which declined later, while the control group presented no response.