Importin alpha1/Rch1, an importin alpha family member, mediates the nuclear import of karyophilic proteins. The present study reports on a monoclonal antibody (MAb) directed against mammalian importin alpha1/Rch1, which was produced by the hybridization of mouse myeloma cells with lymph node cells of an immunized rat. The MAb 1A6 specifically recognized importin alpha1/Rch1 among the importin alpha isoforms, as evidenced by Western blotting. Furthermore, 1A6 detected importin alpha1/Rch1 in HeLa cells by immunofluorescence. This MAb will be useful in immunolocalization and immunoblotting experiments, conducted on different tissue types, to determine the levels of expression of importin alpha1/Rch1 throughout development, as well as further analyses of the biological function and cellular dynamics of this protein.