Recent evidence suggests that Ia+ Langerhans cells may be a primary target for destruction in cutaneous graft-versus-host disease (GVHD). Although it is generally accepted that T lymphocytes with helper/inducer phenotype are essential, the identity of the effector cells is still controversial. We therefore investigated whether a variety of la-reactive cloned helper T cells with different cross-reactivities and functions in vitro can induce cutaneous GVHD following intradermal inoculation into the footpad of the appropriate recipients, whose la antigens are able to stimulate the T cells to proliferate in vitro. All cloned T cells tested caused significant footpad swelling in their appropriate recipients with a course typical for local cutaneous delayed-type hypersensitivity (DTH) reactions. Two of these cloned T cells, SK 1 and BBS, induced local histologic changes consistent with grades 2–3 of cutaneous GVHD in the appropriate allogeneic or syngeneic recipients at 48–72 hr after their intradermal inoculation. Immunohistochemical studies using monoclonal antibodies demonstrated that not only injected cloned T cells but also Lyt-1+ cells derived from the recipient migrate into the epidermis and are responsible for the destruction seen in cutaneous GVHD. In epidermis in which cutaneous GVHD had been induced, expression of Ia by keratinocytes and the damage of Ia+ LC were observed. These results suggest that Ia+ LC and Ia+ keratinocytes may play an important role in the infiltration of la-reactive T cells responsible for cutaneous GVHD.