Abstract: The healthy conventional cattles were inoculated subcutaneouly with different CFU of Brucella abortus strain, the cattles were necropsy on day post inoculation. Tissues were collected for preparing tissue sections and immunohistochemistry protocol was developed to detect distribution of B.abortus bacterium in tissue samples. Results showed that in liver, spleen, kidney and lymphoid tissues were positive for B.abortus antigen. Positive signals also presented in lung, mandibular lymph node. Other tissues such as heart and control were negative for antigen. B.abortus antigens were localized predominately in the cytoplasm of the infected cells and rarely observed in the nucleus of the cells.

Key words: Brucella abortus; immunohistochemical; positive signals