Polymerase basic protein 2 (PB2), a component of the influenza virus polymerase complex, when expressed alone from cloned cDNA in the absence of other influenza virus proteins, is transported into the nucleus. In this study, we have examined the nuclear translocation signal of PB2 by making deletions and mutations in the PB2 sequence. Our studies showed that two distant regions in the polypeptide sequence were involved in the nuclear translocation of PB2. In one region, four basic residues (K-736 R K R) played a critical role in the nuclear translocation of PB2, since the deletion or mutation of these residues rendered the protein totally cytoplasmic. However, seven residues (M K R K R N S) of this region, including the four basic residues, failed to translocate a cytoplasmic reporter protein into the nucleus, suggesting that these sequences were necessary but not sufficient for nuclear translocation. Deletion of another region (amino acids 449 to 495) resulted in a mutant protein which was cytoplasmic with a perinuclear distribution. This novel phenotype suggests that a perinuclear binding step was involved prior to translocation of PB2 across the nuclear pore and that a signal might be involved in perinuclear binding. Possible involvement of these two signal sequences in the nuclear localization of PB2 is discussed.