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The capacity of fresh murine allogeneic bone to induce a specific immune response in vitro was studied. T-cells stimulated by allogeneic bone in vitro were collected and were characterized for state of activation, cell-surface phenotype, and antigen specificity. The stimulating antigens were determined by genetic mapping with use of recombinant inbred strains of mice and by blocking of mixed lymphocyte cultures with use of neutralizing antibodies. Purified T-cells were cultured alone or with allogeneic or syngeneic bone. In some experiments, the bone marrow was removed before in vitro culture. Responding cells were recovered after a secondary exposure to the stimulating bone. Primed cells were used immediately or cell-lines were developed. The data demonstrated that (1) allogeneic bone activated T-cells and induced their proliferation; (2) bone-induced proliferation of T-cells was specific for antigens that map to the major histocompatibility complex of the bone donor; (3) within the major histocompatibility complex, the antigens responsible for proliferation of T-cells were apparently class-I and class-II determinants; (4) removal of bone-marrow cells had no effect on the ability of that bone to stimulate alloreactivity; and (5) all of the alloreactive T-cells had the cell-surface phenotype Thy-+ CD8+ CD4-.