Contact hypersensitivity (CH) has served as a useful model for investigating the allergen-specific immune responses of T cells and skin-associated antigen-presenting cells. We examined the distinct role between CD80 and CD86 on hapten-induced CH in both an induction and an effector phase. Intraperitoneal injection of mAb against CD86, but not CD80, 2 h before sensitization with epicutaneous application of dinitrofluorobenzene led to an almost complete inhibition of ear swelling, and histologically a marked reduction of edema, inflammatory polymorphonuclear cells and lymphocyte infiltration in the dermis. In contrast, the administration with either anti-CD80 or CD86 mAb 2 h before challenge partially inhibited CH reactions and a combination of both mAb did not improve the inhibitory effect. Although Langerhans cells (LC) expressing MHC class II were observed in both the epidermis and dermis 24 h after primary sensitization, CD86(+) LC were observed only in the subepidermal regions and CD80-bearing cells were not detected. Dendritic cells (DC) expressing both CD80 and CD86 were preferentially observed in the T cell areas of draining lymph nodes 24 h after challenge. The administration of anti-CD86 mAb in the induction phase prominently reduced the up-regulation of CD80 and CD86 on DC in the lymph nodes. The predominant role of CD86 was further supported by a marked reduction in proliferation of lymph node T cells against the sensitized hapten after the anti-CD86 treatment. These results suggest that CD86 plays a critical role in the initiation of primary immune responses in the skin, while CD80 and CD86 are not essential in the effector phase of CH reactions.