The effect of intramuscular administration of marbofloxacin at a dose of 2 mg/kg b.wt on humoral immune response was investigated in twenty Newzealand white rabbits of 3-4 months old and weighing about 2-2.5 kg b.wt. Rabbits were vaccinated with the inactivated rabbit viral hemorrhagic disease (RVHD) vaccine. The haemagglutination inhibition (HI) antibody titers, total serum protein; albumin; and globulin levels, and the percentage of each protein fraction were evaluated. The results showed that the administration of marbofloxacin in vaccinated group induced a significant decrease in the HI antibody titers, total serum protein level, globulin level, and gamma globulin percentage concurrently with a significant increase in alpha and beta globulins percentages only at the 1st week post vaccination and treatment. On the other hand, no significant changes could be detected in the total serum protein; albumin; and globulin levels, alpha, beta and gamma globulin percentages of nonvaccinated treated group. Therefore, it was concluded that administration of marbofloxacin is not recommended to be used in vaccinated rabbits as it possess an immunosuppressive effect but can be used safely in non-vaccinated rabbits.