The immuno-inflammatory responses to titanium miniplates used in the treatment of mandibular fractures were studied immunohistochemically at light and electron microscope levels. Titanium miniplates were stably situated on the cortical bone surface. In the soft tissue adjacent to the surface of titanium miniplates, double layered connective tissue was observed, which consisted of dense fibrous connective tissue, and relatively loos connective tissue contained proliferated blood vessels with hypertrophied endothelial cells. These vascular endothelial cells expressed HLA-DR, CD54 and CD62P antigens. In some cases they were CD62Epositive. CD68+ and CD11c+ round or spindle-shaped macrophages had infiltrated around the small vessels. Fine titanium particles were observed in the cytoplasm of these macrophages. Both CD4+ and CD8+ T lymphocytes had also infiltrated around venules in some cases. They were CD4+ T lymphocyte-dominant. Immunoelectron microscopically, CD68+ and CD11c+ macrophages contained titanium particles in the lysosomes. Most of the macrophages showed varying degrees of degenerative change. The presence of titanium was confirmed by energy-dispersive X-ray analysis.