Summary The polyclonal cytokine‐induced killer (CIK) cells exhibit potent cytotoxicity against a variety of tumour cells including autologous and allogeneic acute myeloid leukaemic (AML) targets. At maturity, three lymphocyte subsets: CD3− CD56+, CD3+ CD56− and CD3+ CD56+, constitute the bulk of the CIK cell culture. The CD3− CD56+ subset behaves like classical natural killer (NK) cells where cytotoxicity is potentiated by blocking the human leucocyte antigen Class I molecules in the AML targets. Both the CD3+ CD56+ and CD3+ CD56− subsets, though known to kill autologous and allogeneic targets to a comparable degree and therefore non‐major histocompatibility complex (MHC)‐restricted, nevertheless require the presence of the MHC molecule on the target, which interacts with their CD3–T‐cell receptor complex. Although CIK cells are often termed ‘NK‐like’ T cells, we have demonstrated that the well‐characterized NK receptors KIR, NKG2C/E, NKG2D and DNAM‐1 are not involved in the process of AML recognition for the CD3+ CD56− and CD3+ CD56+ subsets. The CD3+ CD56+ and CD3+ CD56− subsets express a polyclonal and comparable TCRVβ repertoire in a Gaussian distribution. The CD3+ CD56+ subset kills AML targets more efficiently than its CD3+ CD56− counterpart because of the presence of a higher proportion of CD8+ cells. The CD3+ CD56+ subset comprise more terminally differentiated late effector T cells that bear the CD27+ CD28− or CD27− CD28− phenotype, with a higher granzyme A content. In comparison, the phenotype of the CD3+ CD56− subset is consistent with early effector T cells that are CD27+ CD28+ and CD62L+, known to be less cytotoxic but possess greater proliferative potential.