Effects of Proteus vulgaris OX19 on the spleen cells of rabbits were investigated. Control group (n = 5) and Proteus treated group (n = 5) of New Zealand male rabbits were used in this study. Bacteria were injected to the rabbits in five days periods with increasing dosages for one month. Thin sections were examined by transmission electron microscope (Jeol 100CXII). Ultrastructural changes were defined in spleen tissue cells due to the antigenic stimulation of bacteria. Spleen cells observed in control group were in normal structure and cells were in close contact with each other. However, spleen cells of Proteus treated group displayed structural changes with regard to the control group in electron microscopic examinations. Chemotaxis of macrophages, forming of pseudopodia and presence of phagocytic vacuoles were observed. Lymphocytes, the major cells of spleen revealed mitotic activity. In addition, chromatin condensation in nucleus and dilatations in perinuclear space were significant. Interactions of lymphocytes and macrophages were noteworthy. (c) 2012 Elsevier Ltd. All rights reserved.