A conjugate of the monoclonal antibody WT1 and ricin A-chain was studied for its suitability for purging marrow of leukemic T cells for autologous transplantation in T cell acute lymphocytic leukemia (T- ALL). The conjugate was powerfully cytotoxic to the human T-ALL cell line, GH1, which expresses the WT1 antigen at a high density. Treatment of the cells with the conjugate at 10(-11) M reduced their rate of protein synthesis by 50%, and the inclusion of 6 mM ammonium chloride in the cultures enhanced the potency of cytotoxic effect by 10–100- fold. Clonogenic assays indicated that less than 0.1% of GH1 cells survived 3-hr exposure to the conjugate in ammonium chloride. WT1 alone did not react with multipotent (CFU-GEMM) hematopoietic progenitors in normal human bone marrow, as measured by fluorescence-activated cell sorting. Under conditions giving maximal killing of GH1 cells, there was no toxicity to multipotential progenitors in normal human marrow.