CpG DNA are potent immunostimulator and currently being tested as adjuvant in immunotherapy of various diseases. THE AIM of this study was to investigate the effects of a single dose of bacterial CpG DNA challenge on response of murine immune system organs and on a zeta-potential of different lymphoid organ cells and peritoneal macrophages in normal and tumor-bearing mice. METHODS: Indexes and cellularity of immune organs of mice were evaluated. Z-potential was measured by cellular electrophoresis. RESULTS: Subcutaneous administration of CpG DNA induced local transitory hyperplasia of lymph nodes, moderate increase of splenic index and total spleen cellularity, whereas intraperitoneal injection of CpG DNA caused full-blown spleen enlargement and increase of splenocyte content, and also mice showed transitory aseptic peritonitis. Response of thymus on challenge of CpG DNA was bi-phase: initial phase - response on antigen as stimulus, and late proliferative phase. It was noted that tumor growth does not affect zeta-potential in peritoneal macrophages and mononuclear lymphocytes, but causes increase of zeta-potential on thymocytes and decreases it in lymphocytes from lymph nodes. Furthermore, single administration of CpG DNA normalizes of thymocytes and lymph nodes lymphocytes zeta-potential and increases it in peritoneal macrophages and mononuclear lymphocytes. CONCLUSION: The findings demonstrate a close correlation between the hyperplasia of lymphoid follicles induced by challenge of CpG DNA and increase of their cellularity. Observed Z-potential alterations of immune system cells after CpG DNA immunization evidence on more significant polyanion accumulation on the surface of splenic macrophages and mononuclear cells.