The interleukin 2-diphtheria toxin-related fusion protein (IL-2-toxin) rapidly inhibits protein synthesis in IL-2 receptor (IL-2R)-bearing phytohemagglutinin-activated T cells but transiently stimulates DNA synthesis. At 7 hr after interaction with IL-2R+ phytohemagglutinin-activated T cells, IL-2-toxin-treated cells bear augmented steady-state levels of c-myc, interferon gamma, and IL-2R mRNA; these effects are indistinguishable from those produced by recombinant IL-2. Amplification of IL-2 sequences by the polymerase chain reaction reveals an increased level of IL-2 mRNA in cell cultures treated with recombinant IL-2, IL-2-toxin, and cycloheximide. These results suggest that IL-2-toxin can affect de novo IL-2 gene transcription/mRNA stabilization through independent mechanisms exerted by both the IL-2R binding domain and ADP-ribosyltransferase activity of the fusion protein. After 20 hr of culture, IL-2R mRNA was markedly decreased in both IL-2-toxin- and cycloheximide-treated phytohemagglutinin-activated T cells. Although interaction of IL-2-toxin with IL-2R+ T cells initially mimics the stimulatory effects of IL-2 upon c-myc, interferon gamma, IL-2R, and IL-2 gene expression, the consequences of inhibition of protein synthesis mediated by the ADP-ribosyltransferase activity of the toxin dominate after 7 hr and are indistinguishable from those effects mediated by cycloheximide.