Little is known on the effector mechanisms inducing the cutaneous lesions observed during acute graft-vs.-host disease (aGvHD) after allogeneic bone marrow transplantation (BMT). Histological findings have indicated that infiltrating CD8+ lymphocytes probably play a role. We addressed the question of whether host minor histocompatibility (mH) antigen-reactive cytotoxic T lymphocytes (CTL) could account for this phenomenon via direct lysis of the epidermal cell layer. Six CTL clones, obtained from peripheral blood lymphocytes of patients suffering from aGvHD, each recognizing a well-characterized MHC class I-restricted mH antigen epitope, were tested on cultured keratinocytes of nine MHC and mH antigen-typed donors. Four of six mH antigen-specific CTL clones lysed unstimulated MHC class I-expressing, as well as recombinant interferon-gamma (rIFN-gamma)-activated, ICAM-1, MHC class I- and II-expressing keratinocytes. Two strongly cytolytic CTL clones showed no recognition of keratinocytes of donors whose phytohemagglutinin-activated T cell lines were readily lysed. With respect to a GvHD, the results imply that some class I-restricted CTL obtained from peripheral blood lymphocytes of a GvHD patients have the in vitro potential to destroy resting as well as IFN-gamma-activated epidermal cells, whereas others do not. In other words, CTL-defined human mH antigens vary with respect to their expression in the skin. It is intriguing that those minor H antigens which cannot be detected on human keratinocytes in vitro are those known to be associated with the occurrence of GvHD.