We prepared a monoclonal antibody (MAb), Lt-4, which recognizes a tax protein expressed in HTLV-I-infected cells. Indirect immunofluorescence staining showed that the antigen recognized by Lt-4 MAb located mainly in the nuclei of HTLV-I-infected T cell lines such as HUT102, TCL-As2, MT-1 and MT-2, and also weakly in the cytoplasm of MT-2 cell line. Lt-4 MAb did not react with two HTLV-I-uninfected T cell lines tested and fresh and PHA-activated peripheral blood lymphocytes (PBL) of several normal donors. Furthermore, Lt-4 MAb stained the nuclei of HeLa cells infected with a recombinant vaccinia virus encoding the tax but not with a wild type vaccinia virus. In Western blot (WB) and radioimmunoprecipitation analyses, Lt-4 MAb detected a 40 kDa molecule (p40) in HUT102 and TCL-As2 cells, and p40 and p68 in MT-2 cells. These results show that Lt-4 MAb is highly specific for the tax protein.