The ribosome-inactivating protein saporin, from Saponaria officinalis, was coupled by a disulfide bond to monoclonal anti-Thy 1.1 antibody (OX7) and to its F(ab')2 fragment. The immunotoxins were at least as toxic as the plant toxin ricin to the Thy 1.1-expressing cell lines AKR-A and BW5147 in tissue culture. They reduced the rate at which the cells incorporated [3H]leucine into protein by 50% at cell concentrations of 1.5-3 X 10(-11) and 3 X 10(-12) M, respectively. The toxic effect was specific. No toxicity was seen when the immunotoxins were applied to Thy 1.2-expressing EL 4 lymphoma cells at 3 X 10(-8) M, and a control immunotoxin made from an antibody (R10) of irrelevant specificity was without effect on AKA-A cells. Further, the treatment of spleen cells from AKR mice with OX7-saporin at 10(-8) M abolished their response to the T-lymphocyte mitogen concanavalin A, without impairing their response to the B-lymphocyte mitogen lipopolysaccharide. A single iv injection of OX7-saporin into nu/nu randombred mice bearing peritoneal AKR-A lymphoma cells prolonged the survival time of the animals by an extent corresponding to that expected if 99.999% of the tumor cells had been eradicated by the immunotoxin. None of the control materials (unconjugated OX7, unconjugated saporin, OX7 plus saporin, or R10-saporin) delayed tumor growth. The OX7 F(ab')2-saporin conjugate was also highly effective as an antitumor agent, although significantly less so than the conjugate made with intact OX7. Unexpectedly, the acute toxicity of saporin to mice (median lethal dose = 6.8 mg/kg) was elevated eightfold to sixteenfold by conjugation to OX7, R10, or OX7 F(ab')2. Histologic examination of recipients of the immunotoxin revealed gross damage to hepatic parenchymal cells and to the white pulp of the spleen, neither of which was caused by unconjugated saporin. Ricin A-chain coupled to OX7 antibody was one hundredfold to one thousandfold less effective than OX7-saporin as an antitumor agent in vivo, although the two immunotoxins were equally cytotoxic to AKR-A cells in vitro.