Several reports have shown that an increase in T-cell receptor gamma/delta-positive T-cells (γδ T-cells) have been observed following bone marrow transplantation. γδ T-cells expanded from peripheral blood mononuclear cells from normal volunteers were used to investigate the function of γδ T-cells in vitro. Peripheral blood mononuclear cells were cultured with synthetic ligand of γδ T-cells, monoethyl phosphates (MEP), for 7 days. MEP specifically expanded γδ T-cells. Expanded γδ T-cells from subject “B” were added to an A anti-B or A anti-C mixed lymphocyte culture (MLC) containing responder cells from subject “A” and irradiated stimulator cells from subjects “B” or “C”. The cultures were harvested on day 6 and tested for cytotoxicity against stimulator-type Con A blasts. γδ T-cells from subject “B” specifically inhibit generation of allospecific cytotoxic T lymphocytes (CTL) in A anti-B MLC. The results indicate that γδ T-cells exhibit veto-type suppression of alloreaction. If the current experiments are also applicable in vivo, γδ T-cells originating from the donor after bone marrow transplantation may inhibit graft rejection by suppressing recipient anti-donor reactivity. γδ T-cells may be involved in the suppression of allogeneic reaction in vivo following allogeneic bone marrow transplantation.