Ricin A chain and pokeweed antiviral protein (PAP), two enzymes that inhibit the action of eukaryotic ribosomes, were coupled by cleavable, N-succinimidyl-3-(2-pyridyldithio)propionate, and noncleavable m-maleimidobenzoyl-N-hydroxysuccinimide ester, cross-linking reagents to monoclonal antibodies directed against Thy 1.1 antigen. Leukemia cells that contained Thy 1.1 antigen were selectively killed compared to Thy 1.2-containing cells. The composition of the conjugates was determined by radioimmunoassay, and most of the immunotoxins contained about equal molar quantities of antibody and ribosomal inhibitor. Ricin A chain linked to antibody by a noncleavable m-maleimidobenzoyl-N-hydroxysuccinimide ester cross-link was not cytotoxic, but PAP coupled to the same antibody was. Both immunotoxins linked by a cleavable disulfide bond were cytotoxic. Disulfide-linked F(ab')2-PAP was cytotoxic, but it was about 45 times less efficient than disulfide-linked IgG-PAP. There was only a 3.2-fold difference in their ability to inhibit ribosomes in vitro. The relative difference between in vitro action and cytotoxicity could be accounted for by differences in the affinity of the immunotoxins to the cell surface. Neither PAP or ricin A chain disulfide linked to a monoclonal antibody against Mr 15,000 envelope protein, a murine leukemia virus coat protein, were not cytotoxic, although both conjugates bound to the cell surface. Because of its stability, ease of purification, and lack of an analogue of ricin B chain, PAP may be more useful than ricin A for immunotoxin synthesis.