Immunostimulating factor (ISTF) isolated from Actinobacillus actinomycetemcomitans which has been described previously, is distinct from lipopolysaccharide and induces proliferation of B cells. This study was undertaken to investigate whether ISTF might enhance the stimulation of other immune cells. Immunohistochemically, ISTF exhibited a profound stimulating effect on macrophages and dendritic cells as well as B cells in the spleen of BALB/c mice. ISTF was also recognized for its capacity to induce direct activation of mouse macrophages to produce IL-6, TNF-alpha, and NO and MHC class II expression. Therefore, it is postulated that ISTF stimulates macrophages and possibly other cells to produce a wide variety of proinflammatory mediators, which may be involved in the chronicity and tissue destruction of periodontal disease.