Mice were exposed to 500ppb formaldehyde (HCHO) daily 8 hr, 6 days in week for 6 weeks. Trimelitic acid (TMA) and oval albumin (OVA) for type I allergy, and oxazolone (OXZ) for type IV allergy were employed as allergens to induce each type of allergic responses. Local lymph node assay (LLNA) were performed using TMA or OXZ for evaluation of the sensitizing phase. For evaluation of the arising phases, followed methods were employed. Oval albumin (OVA) was treated repeatedly inter peritonea cavity (ip) or nasal cavities, for evaluation of IgE production. OXZ were painted on back of ear at day 5 after the sensitization and observed thickness, histology and mRNA expression in ear for evaluation of delayd type allergic response. In LLNA with TMA,increase of cell number and enhancement of cell proliferation were observed dominantly in exposed mice. LLNA with OXZ,those were observed equal level in control and exposed mice. Anti-OVA IgE titer elevated slightly with ip sensitization but strongly reduced with nasal cavities sensitization in exposed mice. Exposed mice showed early peak of ear swelling compare control mice. In histological observation, exposed mice ear showed low infiltration of inflammatory cells and sever edema, although high cell infiltration and moderate edema in control mice. Expression of type IV allergy relating cytokine mRNA was suppressed in ear tissues of exposed group, but that of type I allergy was unchanged. Our data suggest that HCHO modulates allergic responses ; HCHO exposure may accelerate type I allergy and moderate type IV allergy in both phases of sensitization and arising of allergy symptoms. Those findings mean HCHO accelerates sensitization and symptoms of atopic diseases.