Interferon-alpha/beta-inducible Mx proteins belong to the family of large GTPases and share high sequence homology with dynamins in their N-terminal GTP-binding domains. In addition, Mx proteins have a conserved C-terminal leucine zipper element that is involved in their oligomerization. Cytoplasmic human MxA protein mediates resistance to multiple RNA viruses, whereas no antiviral activity has been found for human MxB protein. We have previously shown that MxB protein exists as a nuclear 78-kDa and as a cytoplasmic 76-kDa form in interferon-alpha-induced human cells. Using various influenza hemagglutinin epitope-tagged MxB gene constructs in transient transfection experiments in COS-1 cells, we show that the cytoplasmic 76-kDa MxB protein forms hetero-oligomers with the nuclear 78-kDa MxB protein via the C-terminal leucine zipper element. This enables the cytoplasmic form of MxB protein to be translocated into the nucleus together with the nuclear form of MxB protein. This finding was confirmed in interferon-alpha-induced HEp-2 and T98G cells transfected with various MxB gene constructs. Cell fractionation studies also suggest that a considerable amount of the cytoplasmic MxB protein is also found in the nucleus. Using confocal laser microscopy, we also demonstrate that the cytoplasmic MxA and the nuclear MxB proteins do not colocalize/oligomerize with each other, and both of these proteins are retained in their specific cellular compartments.