Activation of T lymphocytes by Ags or cytokines results in translocation of the transcription factors NF-kappa B, AP-1, NFAT, and STAT from the cytoplasm into the nucleus. The first step in the nuclear import process is recognition of a nuclear localization sequence (NLS) within the karyophilic protein by a cytoplasmic receptor such as the importin (karyopherin)-alpha subunit. The NLSs of NF-kappa B, AP-1, and NFAT differ and the NLS of STAT1 has not yet been identified. Herein we demonstrate that the inducible nuclear import of NF-kappa B, AP-1, NFAT, and STAT1 in Jurkat T lymphocytes is significantly inhibited by a cell-permeable peptide carrying the NLS of the NF-kappa B p50 subunit. NLS peptide-mediated disruption of the nuclear import of these transcription factors results in inhibition of I kappa B alpha and IL-2 gene expression, processes dependent on NF-kappa B or the combination of NF-kappa B, AP-1, and NFAT. Further, we show that inhibitory NLS peptide interacts in vitro with a cytoplasmic NLS receptor complex comprised of the Rch1/importin (karyopherin)-beta heterodimer expressed in Jurkat T cells. Taken together, these data indicate that the inducible nuclear import of NF-kappa B, AP-1, NFAT, and STAT1 in Jurkat T cells can be regulated by NLS peptide delivered noninvasively to the cytoplasm of Jurkat T cells to target members of the importin (karyopherin)-alpha beta NLS receptor complex.