In resting cells NF-κB transcription factors are retained in the cytoplasm as latent inactive complexes, until they are activated and rapidly transported into the nucleus. We show that all NF-κB proteins are imported into the nucleus via a subset of importin α isoforms. Our data indicate that the NF-κB components of the classical and alternative pathways have somewhat different specifities to importin α molecules. Based on the results from binding experiments of in vitro-translated and Sendai virus infection-induced or TNF-α-stimulated endogenous NF-κB proteins, it can be predicted that the specifity of NF-κB proteins to importin α molecules is different and changes upon the composition of the imported dimer. p52 protein binds directly to importin α3, α4, α5 and α6 and c-Rel binds to importin α5, α6 and α7 via a previously described monopartite nuclear localization signals (NLSs). Here we show that RelB, instead, has a bipartite arginine/lysine-rich NLS that mediates the binding of RelB to importin α5 and α6 and subsequent nuclear translocation of the protein. Moreover, we show that the nuclear import of p52/RelB heterodimers is mediated exclusively by the NLS of RelB. In addition, we found that the NLS of p52 mediates the nuclear import of p52/p65 heterodimers.