Interleukin-2 administration induces CD4 T cell expansion in HIV-infected patients, however, toxicity can limit dosing. BAY 50-4798 is a recombinant IL-2 analog with >1000-fold specificity for the high-affinity IL-2 receptor. The effects of this compound on unstimulated human PBMC were evaluated. PBMC from HIV(-) and HIV(+) donors were cultured in vitro with incremental doses of BAY 50-4798 or aldesleukin. CD25 expression and proliferation were evaluated with flow cytometry. Cytokine levels were measured by ELISA in culture supernatants. BAY 50-4798 induced dose-dependent increases in CD25 expression and proliferation of T cells, NK, and B cells and showed selectivity for CD4 T cells expressing CD25. Induction of pro-inflammatory cytokines was also dose-dependent and was observed at the concentrations of BAY 50-4798 with the highest biologic activity. These data suggest that BAY 50-4798 can induce proliferation of unstimulated T cells but loss of T cell selectivity and induction of pro-inflammatory cytokines occur at concentrations exerting the highest biologic activity.