Abstract The struture of the 38 kD cell surface glycoprotein identified by the monoclonal antibody MOv18 and specifically expressed by human ovarian carcinomas has been investigated at a molecular level. The ovarian carcinoma cell line IGROV-1, which expresses high levels of Ca-MOv18, was treated with the phosphatidylinositol-specific phospholipase C from B. thuringiensis. The phospholipase C specifically released most of the Ca-MOv18 molecules as shown by flow cytometric analysis of the treated cells and by radioimmunometric assays of the corresponding supernatants. Consistent with the known structure of other phosphatidylinositol-linked molecules, Ca-MOv18 was biosynthetically labeled by [ 3H]ethanolamine and the labeled molecules were immunoprecipitated from the supernatant of the phospholipase C treated cells. Evidence that Ca-MOv18 is anchored to the cell membrane via phosphatidylinositol may prove to be relevant in current investigations regarding the biological and clinical significance of this tumor marker.