Productive infection of cells by human immunodeficiency virus type 1 (HIV-1) is associated with the activation state of the cell and its obligatory expression of the interleukin-2 receptor (IL-2R), the latter providing a new target for antiviral therapy. A quantitative RNA-RNA hybridization assay is employed to detect production of HIV-1 RNA and to show that two IL-2 diphtheria toxin-related fusion proteins (DAB486IL-2 and its more potent, truncated form DAB389IL-2) inhibit HIV-1 RNA production in infected cells. A mutant form of DAB486IL-2 containing a single point mutation that inactivates the adenosine diphosphate ribosyltransferase activity of the toxin does not inhibit HIV RNA production, even though the molecule binds to the IL-2R. The active fusion proteins inhibit viral RNA replication in cells infected with HIV-1 clinical isolates as well as with a ZDV-resistant strain of HIV-1. These results indicate that IL-2 receptor-targeted fusion proteins can be utilized to inhibit HIV-1 replication effectively in infected human lymphocytes.