A murine monoclonal antibody (MAb) 2G3 of the IgG1 type was raised using the human esophageal squamous cell carcinoma (SCC) cell line TE-2. Immunoblotting with 2G3 indicated that the antigen recognized by 2G3 has a molecular weight of 34 kD. Its activity was evaluated by immunoperoxidase and immunofluorescence on frozen and paraffin sections of various normal tissues, normal and benign tumors as well as various established cell lines. The pattern of reactivity revealed that the antigen recognized by 2G3 was expressed mainly by esophageal SCC. The only exception was represented by malignant breast tumors, where it reacted weakly. Scatchard analysis using 125I-labelled 2G3 showed that TE-2 has approximately 7.5 times more binding sites than the MCF-7 breast cancer cell line. The use of this new MAb is therefore proposed for the histopathological diagnosis of esophageal SCC.