We studied early NK-cell recovery in 29 allografted patients undergoing different lymphoreductive regimens. Already at 2 wk after graft take, the number of NK cells had reached (supra)normal levels but NK-cell subsets were skewed. The number of CD56(dim) CD16(bright) NK cells was low and correlated strongly with the level of hematopoiesis, whereas the number of the more abundant NK cells expressing high levels of CD56 did not. Post-transplant CD56(bright) NK cells (ptCD56(bright)) differed from CD56(bright) NK cells in normal controls (CD56(bright)) in being HLA-DR- and perforin-positive, CCR7(-), CD27(-), CD127(-) and mostly c-kit(-). CD56(bright) from normal controls stimulated by IL-15 in vitro (NK(IL-15)) acquired all the characteristics distinguishing CD56(bright) from ptCD56(bright). IL-2 exerted similar effects. Moreover, when cultured without cytokines, ptCD56(bright), CD56(bright) and NK(IL-15) responded similarly by upregulating CD127 and c-kit but not CCR7. IL-12 stimulated IFN-gamma production in ptCD56(bright), whereas CD56(bright) responded only to IL-12 plus IL-15. Hence, ptCD56(bright) have all the features of cytokine-stimulated CD56(bright). Because only patients with low numbers of T cells had high numbers of ptCD56(bright), we conclude that ptCD56(bright) are activated CD56(bright) that expand while competing with T cells for the elevated post-transplant level of IL-15.