Effect of hIL-15 and sIL-15Rα administration on total numbers of (A) NK1.1+ cells, (B) CD3+/CD8+ cells in the spleens of treated mice. C57BL/6/J male mice (n = 4 per group) were dosed once per day for three consecutive days with recombinant proteins as indicated. 24 h post treatment spleens were extracted and the total number of NK1.1+ CD3+ and CD8+ cells measured according to Materials and Methods. hIL-15 alone and pre-associated IL-15/sIL-15Rα complex significantly increased numbers of NK1.1+ cells in the spleen compared with PBS-treated animals. Administration of hIL-15 followed by sIL-15Rα 1 h apart at separate sites caused a significant increase in numbers of NK1.1+ cells compared to PBS-treated animals. *P < 0.05, **P < 0.01, ***P < 0.001. hIL-15, human interleukin-15; hIL-15/sIL-15Rα, pre-associated complex of human IL-15 and soluble IL-15 receptor α; IL-15, interleukin-15; PBS, phosphate-buffered saline.