This study was performed to investigate the effects of Immunomodulating factor (IMF), derived from Actinobacillus actinomycetemcomitans, on various immune cells in the mouse spleen. A single dose of IMF (10 microgram/kg) was administer-ed to BALB/c mice by intraperitoneal injection. After the mice were sacrificed in groups of five at 6 h and 24 h, the spleens were removed. The immunocytochemical characterization of the immune cells was carried out using the various monoclonal antibodies in cryostat-cut sections. We demonstrated in this study a strong stimulating effect of IMF on dendritic cells and B lymphocytes in the mouse spleen after IMF administration. The MOMA-1(+) immunoreactivity on the marginal metallophilic macrophages in the splenic marginal zone disappeared 6 h and reappeared 24 h after IMF treatment. However, various subpopulations of T lymphocytes, CD3(+), CD4(+), CD8(+), TCRalpha, beta(+) and Vbeta8(+) T cells in the mouse spleen did not show any significant change in their distributional pattern after IMF treatment. Dendritic cells were found to be increased in number in the periarterial lymphatitc sheath, and B lymphocytes were also increased in number in the lymphoid follicles of the spleen after IMF injection. In conclusion, IMF exhibited a potent stimulative effect on dendritic cells and B lymphocytes in vivo.