The expression of placental alkaline phosphatase in 116 ovarian epithelial tumours was examined in formalin-fixed tissues used for routine histopathologic examination. In the total material, 51% of the tumours displayed positive immunoreactivity, as described by the monoclonal anti-placental alkaline phosphatase antibody C2, with similar incidence (46-67%) in the four major groups of the adenocarcinomas, i.e., serous, mucinous, endometrioid and mesonephric tumours. By use of a histochemical staining index the mucinous and mesonephric tumours demonstrated a more intense staining (2.1 and 2.6) compared to the serous and endometrioid tumours (0.9 and 1.5). The relevance of the findings is discussed in relation to the use of monoclonal antibody technologies for radioimmunolocalization and radioimmunotherapy.