During MHC-matched bone marrow transplantation, a graft-versus-host reaction (GVH) mediated by donor T lymphocytes and directed against host minor histocompatibility antigens may be observed. The establishment of T cell clones from GVH mice has enabled us to study minor antigens recognized during the reaction. Most of the T cell clones were Thyl+CD4+CD8-alpha beta+. Four minor, weakly polymorphic antigens, were evidenced; two of those antigens, recognized by CD4+ T cell clones, were implicated in GVH. One CD8+ T cell clone cannot provoke GVH mortality. The two "killer" clones express V beta 18 and V beta 2 TCR gene segments, respectively.