BACKGROUND Until now, cytokine-induced killer (CIK) cells were assumed to be part of the type I natural killer T (NKT) cell population, but it was not yet investigated if this is correct. METHODS For analysis, CIK cells were generated by various culture conditions. Human type I NKT cells express a T cell receptor (TCR) composed of an invariant V alpha 24-J alpha Q chain combined with one of several V beta chains. The V alpha 24 is a reliable marker for the presence of these TCRs. RESULTS While comparing cultures stimulated with different substances, we observed the lack of any V alpha 24 on the surface of CIK culture cells. CONCLUSIONS We conclude that CIK cells do not belong to the type I NKT cells.