Repeated administration of antigen often leads to consequences different from those expected with fewer encounters with the antigen, but little attention has been paid to the effects of repeated epicutaneous application of antigens. To investigate whether repeated epicutaneous application of a contact-sensitizing agent that is generally thought to evoke a typical delayed-type hypersensitivity response could result in adverse or different consequences, BALB/c mice were sensitized with 2,4,6-trinitro-1-chlorobenzine and then were repeatedly elicited on the original sensitized site with the same antigen for 24-48 d. Detailed analyses showed that the time-course of antigen-specific hypersensitivity responses shifted from a delayed-type hypersensitivity to an immediate-type response followed by a late reaction as epicutaneous applications were repeated, a finding different from that previously reported. Development of these hypersensitivity responses was antigen specific, and this shift was associated with epidermal hyperplasia, accumulation of large numbers of mast cells and CD4+ T cells beneath the epidermis, and elevated serum levels of antigen-specific IgE. The immediate-type response to 2,4,6-trinitro-1-chlorobenzine was also induced in 2,4,6-trinitro-1-chlorobenzine-treated, genetically mast cell-deficient W/Wv mice that contained significant numbers of mast cells, but not in similarly treated S1/S1d mice devoid of mast cells. Our experimental system would provide a simple, reproducible animal model for chronic skin inflammation induced by various antigens.