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A murine monoclonal antibody, AD117m, was produced by fusion of murine myeloma cells with spleen cells obtained from mice immunized with intact OMC-4 (a human adenocarcinoma cell line derived from uterine cervix). The hybridoma that produced AD117m was selected from five colonies showing strong reactivity with OMC-4 but no reactivity with OMC-1 (a human squamous cell carcinoma cell line from uterine cervix). Since antigenic activity was reduced by treatment of the cells with periodate and enhanced by treatment of them with sialidase, the epitope of the antigen seemed to be expressed as a carbohydrate moiety. The AD117m-defined antigen was found not only in the glycoproteins and glycolipids of OMC-4 cells but also in a human meconium neutral glycolipid fraction. We purified the antigen from glycolipids of human meconium, and its structure was characterized by methylation analysis, mass-spectrometry, and sequential glycosidase treatment. The antigen structure was concluded as Galβ1-3GlcNAcβ1-3Galβ1-4Glcβ1-1Cer (lactotetraosylceramide). Immunohistochemical study using formalin-fixed and paraffin-embedded tissue sections showed that in the uterine cervix, all cases of adenocarcinomawere stained with AD117m (20/20), but AD117m reacted in only two out of ten cases of squamous cell carcinoma. In an effusion cytological study, AD117m was found to distinguish metastatic carcinoma cells from reactive mesothelial cells, and showed increased specificity and sensitivity when compared with antiCEA antibody. These observations indicate that AD117m is able to improve the differential diagnosis between reactive mesothelial cells and metastatic carcinoma cells in effusion cytology.