Twenty-three monoclonal antibodies (MAbs) against the IL-2 receptor alpha-chain (CD25) were evaluated as ricin A-chain immunotoxins for the treatment of Hodgkin's disease. Primary screening used an indirect assay in which the cells were treated with the test antibody followed by a Fab' immunotoxin against mouse immunoglobulin. This screening identified 5 MAbs which inhibited protein synthesis in L540 Hodgkin cells by 50% at a concentration (IC50) of 6 x 10(-11) M or less: RFT5 gamma 1, RFT5 gamma 2a, B-B10, B-F2 and B-G3. These MAbs were then linked directly to deglycosylated ricin A-chain (dgA) and were confirmed to have potent and specific toxicity for L540 cells. The immunotoxins had the following potency order: RFT5 gamma 1 greater than RFT5 gamma 2a greater than B-B10 greater than B-F2 greater than B-G3. The most effective immunotoxin, RFT5 gamma 1.dgA, had an IC50 value of 7 x 10(-12) M, which is the same as that of whole ricin. In vivo, a single intravenous injection of 48 micrograms of RFT5 gamma 1.dgA, RFT5 gamma 2a.dgA, B-B10.dgA or B-F2 induced lasting complete remissions in 78, 66, 50 and 44%, respectively, of nude mice bearing subcutaneous solid L540 tumours of 0.7 cm diameter. Two tumours which regrew after B-B10.dgA treatment were re-established in tissue culture. Both had reduced sensitivity to B-B10.dgA in vitro but not to immunotoxins recognizing different antigens on Hodgkin cells. The MAbs that produced the most potent immunotoxins, RFT5 gamma 1, RFT5 gamma 2a and B-B10, had no significant cross-reactivity with normal human tissues outside the lymphoid system as judged from indirect immunoperoxidase staining of frozen sections. By contrast, B-F2 strongly stained normal human renal tubules.