Lymphocyte activation induces or increases the expression of several surface structures, none of which is characteristic of an activated cell subset. In particular, structures such as CD45RO, CD25, CD26, CD49b, CD54, CD71 are expressed by the vast majority of lymphocytes at various times following in vitro activation. CD39 molecules were originally identified on activated B lymphocytes and have recently been described on activated T cell clones. In the present report, we have characterized phenotypically and functionally defined cell subsets generated during an in vitro allostimulation. Results indicated that the percentage of CD39+ cells reached a maximum at day 6 and remained stable thereafter. We demonstrate that CD39 expression allows the identification within the allosensitized CD8+ cytotoxic cells of distinct subsets of cells mediating allo cytotoxic T lymphocyte or natural killer (NK)‐like reactivity. More precisely, CD8+CD39+ alloactivated cells mainly mediate specific killer activity, whereas CD8+CD39− alloactivated cells predominantly exhibit NK‐like reactivity. Further, we show a high functional correlation associated with the lack of CD39 expression on NK‐like alloactivated CD8+ cells, while there is no association with CD56 or CD57 NK‐associated structures.