CD56^dim NK cells proliferate less than CD56^bright NK cells. Enriched NK cells were sorted into CD56^bright KIR⁻, CD56^dim KIR⁻, and CD56^dim KIR⁺ NK cells and assayed for proliferation after 14 days of culture with the mouse stromal cell EL08–1D2 and exogenous IL-15 (n = 4). NK cells were either in direct contact with the stromal cells (black bars: EL Contact) or separated from them by a Transwell membrane (hatched bars: EL TW). These data are shown in the lower part of the figure: CD56^bright KIR⁻ NK cells proliferated more than either CD56^dim subset (*P < .01) and the CD56^dim KIR⁻ population proliferated more than CD56^dim KIR⁺ cells (P = .001). In the experiment shown in the boxed, upper part of the figure the CD56^dim KIR⁻ subset was further divided into CD56^dim NKG2A⁻KIR⁻ and NKG2A⁺KIR⁻ subsets (n = 4) and tested in a 6-day thymidine incorporation assay where the NKG2A⁻KIR⁻ subset showed less short-term responsiveness to IL-15 (10 ng/mL) (P = .002). Error bars indicate SEM.