Interleukin 2-dependent alloreactive cytotoxic T cell lines, with activity predominantly directed against the HLA-A2 antigen, have been generated in vitro by stimulating blood mononuclear cells from donors nonimmune to the Epstein-Barr (EB) virus with appropriate numbers of EB virus-transformed B cells from A2-homozygous individuals. Such effector cells were tested against a panel of EB virus-transformed target cell lines all expressing the serologically defined A2 antigen but typed into "common A2" and "variant A2" subgroups on the basis of their recognition by A2-restricted EB virus-specific cytotoxic T cells. "Variant A2" responder cells cocultivated with "common A2"-bearing stimulators gave rise to effector T cell lines which recognized only the "common A2"-bearing subgroup of targets. By contrast, responder cells from A2-negative donors stimulated with "common A2"-bearing cells produced effector T cell lines in which the strong lysis of "common A2"-bearing targets was accompanied by a lower, but still significant, lysis directed against all targets within the "variant A2" subgroup. In both cases, lysis of the target cells was blocked equally well by the anti-A2-specific monoclonal antibody MA2.1 as by the monoclonal antibody W6/32 specific for HLA-A, -B, and -C determinants. This suggests that HLA-A2 molecules possess at least two distinct sets of epitopes capable of inducing alloreactive T cell cytotoxicity: first, epitopes probably associated with T cell-restricting sites, which generate subgroup-specific responses, and second, epitopes shared by all A2 molecules, and perhaps associated with serologically defined sites, which generate "pan A2" group-specific responses.