The selective cytotoxicity of the xanthine oxidase conjugated to an 8A monoclonal antibody recognizing a human plasma cell-associated antigen has been described. The selectivity and the toxicity of the hypoxanthine/conjugated xanthine oxidase system was increased by removing the excess of conjugate and by adding chelated iron. Under these experimental conditions the cytotoxicity of the conjugate exceeded that of free xanthine oxidase by one order of magnitude. The conjugate effectively purged bone marrow from infiltrating neoplastic plasma cells and added target Raji cells, provided blood was removed and bone marrow peroxidases were exhausted. In conditions of purging effectiveness the conjugate had no toxicity to CFU-GM. No toxicity to mice was observed after i.v. injection of xanthine oxidase-antibody conjugate up to 2.9 U/kg body weight. Thus the hypoxanthine/conjugated xanthine oxidase system could be an effective and nontoxic tool for the ex vivo bone marrow purging in multiple myeloma patients for autologous transplantation.