NF-kB is a eukaryotic transcription regulatory factor. In T cells and T cell lines, NF-kB is bound to a cytoplasmic proteic inhibitor, the IkB. Treatment of T cells with mitogens (phorbol esters) or cytokines (TNF alpha) induces NF-kB nuclear translocation and the subsequent expression of NF-kB dependent T cell genes. Here we examined the activation of NF-kB in human T cell thymic progenitors. We report differences in (Ca2+)i requirement for NF-kB activation in thymocytes as compared to mature T cells. Furthermore, our results indicated that thymocytes have a constitutively active form of NF-kB, suggesting that they are activated in vivo.