Circulating human IL-15 is exclusively present as heterodimeric IL-15/sIL-15Rα complex. Serum samples from lymphodepleted cancer patients were preincubated with exogenous sIL-15Rα to allow the in vitro formation of IL-15/sIL-15Rα complexes from any free IL-15 potentially present in the sample. Serum samples were next evaluated for the amount of sIL-15Rα–associated IL-15 by ELISA. Serum sample S8, S9, or S10 was preincubated with increasing sIL-15Rα concentrations (0-2250 pg/mL; left panel). Serum samples S1, S2, S4, S5, and S7 were preincubated with a fixed amount of sIL-15Rα (750 pg/mL; right panel). The assay was repeated twice.