Proliferative activity induced by the combination of IL-2 and IL-15 on CD3+ (A, n = 9), CD3⁻ (B, n = 5), and normal E+-rosetted peripheral blood lymphocytes (C, n = 4). Data are expressed as cpm × 10⁻³ ± SEM. Fixed concentrations of IL-2 (0.1, 1, 10, and 100 ng/mL) were added to graded doses of IL-15 (0.1, 1, 10, and 100 ng/mL).