The reports of Miyanohara et al. (1986) and Murphy et al. (1986) were the first to describe the genetic construction, expression, and receptor-specific selective toxicity of a chimaeric toxin. In the present report, we have extended these earlier observations and have shown that the fusion of a modified gene encoding IL-2 to a truncated diphtheria toxin gene also results in the expression of a biologically active chimaeric IL-2 toxin. In both instances we have used receptor-binding-domain substitution and have genetically coupled those portions of the diphtheria toxin structural gene that encode the ADP-ribosyl transferase activity of fragment A and lipid-associating domains of fragment B to modified genes which encode either the polypeptide hormone alpha-MSH or the T-cell growth factor IL-2. The chimaeric toxins expressed from these gene fusions have been shown to be selectively targeted to those eukaryotic cells that carry specific surface receptors for the ligand compounds of the hybrid. For example, in the case of the IL-2 toxin, it is clear that the selective action of this hybrid protein is based upon both its diphtheria-toxin and IL-2-related components. Following binding to the IL-2R on activated and/or malignant T-cell, IL-2 toxin is internalized by receptor-mediated endocytosis. Upon acidification of the endosome, diphtheria toxin fragment B portions of the chimaeric toxin facilitate the delivery of fragment A to the cytosol where it catalyses the ADP ribosylation of EF-2. The assembly of chimaeric toxins at the level of the gene offers several advantages over chemical linkage. Since chemical linkage of the toxophore and ligand components of the conjugate toxins requires activation of the epsilon-amino moiety of lysine residues with reagents that will allow for subsequent disulphide linkage, the precise site of coupling is generally not known. In addition, there has been considerable concern over the lability of the disulphide bond between the toxophore and ligand components in vivo due to the action of disulphide reductases. The assembly of chimaeric toxins at the level of the gene allows for precise linkage of the toxophore and ligand components. Since the linkage between the toxophore and ligand is a peptide bond, the chimaeric toxin should be stable in vivo. In addition, the genetic construction of chimaeric toxins also allows for further protein engineering through site-directed mutagenesis.(ABSTRACT TRUNCATED AT 400 WORDS)