In recent years, increased attention has been given to allergy to dental metal materials. It has been assumed that the allergic reaction to metals was cell-mediated type hypersensitivity reaction. But it has not yet been clarified in detail. Several traditional techniques have failed to induce contact allergy to metals in the animal. In this study, for the purpose of inducing the experimental allergy to nickel, rats were sensitized to peritoneal macrophages that engulfed nickel particles, as antigen. Rats were inoculated twice at one week interval with the antigen intraperitoneally. A week after the latest inoculation, the rats were inoculated again with the antigen into the gingiva. The rats were sacrificed on 6, 12 and 48 hours after the gingival inoculation. Using immunoperoxidase labeling technique, we examined the distribution of cells which bound anti-interleukin-2 receptor (IL-2R) monoclonal antibodies in the gingival tissues and submandibular lymph nodes, for the purpose of investigating the mechanism of the expression of allergic reaction to metals. In submandibular lymph nodes, IL-2Rs were not or hardly observed 6 hours after the gingival inoculation. After 12 hours, however, IL-2Rs were slightly observed in medulla and were clearly observed after 48 hours. The result suggested that antigen signal was transmitted to submandibular lymph nodes, IL-2R appeared on the surface of T cells, B cells and macrophages, and then T cells proliferated. In gingival tissues, polymorphonuclear leukocytes infiltrated after 6 hours. After 12 hours, mast cells characteristically were distributed and degranulated. The result suggested that antibody dependent type I hypersensitivity was involved in this reaction. After 48 hours, IL-2Rs were clearly observed. The result suggested that sensitized T cells were distributed in gingival tissues, and the cells responded to the antigen and secreted lymphokines. These results suggested that type I hypersensitivity was involved in early experimental allergic reaction to nickel, and type IV hypersensitivity was predominated after that.