The Jun protein is a transcription factor of the AP-1 complex, and it is concentrated in the cell nucleus. While the cellular Jun protein is transported into the nucleus in a cell-cycle-independent fashion, the oncogenic viral version of the protein translocates into the nucleus most rapidly during the G2 phase of the cell cycle and only slowly during G1 and S phases. This cell cycle dependence of nuclear transport has been mapped to the cysteine to serine mutation in the carboxyl-terminal portion of viral Jun. We have identified a complex nuclear translocation signal located in the basic region of viral Jun. This signal has the sequence ASKSRKRKL. A peptide of this sequence synthesized in vitro and conjugated to IgG can mediate cell-cycle-dependent translocation of the microinjected conjugate from the cytoplasm into the nucleus. The nuclear translocation signal has two functional domains. The pentapeptide RKRKL is sufficient as a cell-cycle-independent nuclear address. The entire signal is needed for cell-cycle-dependent nuclear translocation. The amino-terminal tetrapeptide contains the cysteine to serine substitution responsible for cell cycle dependence. Deletion analysis of the Jun protein suggests that the nuclear translocation signal identified in the basic region is required for nuclear translocation of Jun and may be the only such signal in the Jun molecule.