NF-kappa B/Rel family plays a pivotal role in a wide variety of cellular functions including growth, development, apoptosis and stress responses. Recent studies indicated that NF-kappa B is also involved in the cell cycle regulation, and high expression of c-Rel results in a cell cycle arrest at the G1/S-phase transition (Bash, J., Zong, W,-X., and Gelinas, C. (1997) Mol. Cell. Biol. 17, 6526-6536). Here we report the detection of Cdk2, a critical kinase responsible for the G1/S-phase transition, in immune complexes precipitated by the NF-kappa B antisera. Cdk2 and NF-kappa B association was detected by co-precipitation in the nuclear lysates of the G1/S-phase cells, and was found in cultured cell lines and in T cells purified from human peripheral blood. Using an affinity column containing the C-terminal peptide of human c-Rel, we isolated cyclin E, the regulatory subunit of the Cdk2 complex, as a c-Rel-binding protein. These findings support and provide physical basis for the involvement of NF-kappa B in the G1/S-phase cell cycle control, and suggest an important role played by the C-terminal sequence of c-Rel.