We report on the preparation of an immunotoxin consisting of xanthine oxidase, a free-radical-producing enzyme, covalently linked to an anti-CD3 monoclonal antibody. The immunotoxin retained both enzymic and immunological properties and its toxicity to target cells (a) was greater than that of the free enzyme, (b) was proportional to the enzyme concentration, and (c) was reduced either in the absence of hypoxanthine or by an excess of free anti-CD3 monoclonal antibody. The cytotoxicity and selectivity of the hypoxanthine/conjugated xanthine oxidase system were potentiated by the addition of chelated iron and by washing away the unbound immunotoxin prior to the addition of substrate. The same system was not toxic to bone marrow progenitor cells. A possible use of this immunotoxin for the ex vivo purging of organs to be transplanted from T lymphocytes, to avoid the graft-versus-host reaction, is suggested.