Haploidentical bone marrow transplantation (BMT) is associated with a high risk of severe graft-versus-host disease (GVHD). While pan-T cell depletion of the graft is the most effective means of preventing severe GVHD, it is associated with delayed recovery of T cell function leading to fatal infections. We used two related Pseudomonas exotoxin-based immunotoxins, anti-Tac(Fv)-PE38 and anti-Tac(Fv)-PE38KDEL, that both target the IL-2 receptor on activated T cells, to specifically deplete alloreactive lymphocytes against haploidentical stimulators. The functional capacity of the remaining lymphocytes was tested in proliferative assays against the original haploidentical stimulator and pooled cells from other mismatched donors (third party). We varied the recombinant toxin concentration and schedule to determine the optimum conditions for selective depletion. In 10 experiments, the mean residual reactivity after depletion was 7.6 +/- 1.4% against the haploidentical stimulator and 64.2 +/- 5% against the third party, expressed as a percentage of the undepleted response to the same stimulators. Depletion was shown to be specific for mixed lymphocyte culture (MLC)-activated lymphocytes. The immunotoxin did not affect CFU-GM growth of normal BM cells. This selective depletion of haploidentical alloreactivity could be used to prevent GVHD while conserving immune recovery following haploidentical BMT.