Formation of basement membrane-like substance or so-called collagen core was characteristic of clear-cell carcinoma of the ovary in ascites cytology. The hyaline extracellular material was stained light green in Papanicolaou smears and pinkish to purplish red in Giemsa preparations and was frequently observed within the cancer cell clusters in all ten samples of clear-cell carcinoma. Such a structure termed "raspberry body" was focally seen in one of 30 specimens of serous cystadenocarcinoma and one of 30 samples containing reactive mesothelial cells. The "raspberry body" was not found in ascitic fluid from ten patients with mucinous cystadenocarcinoma and two with endometrioid carcinoma. Overproduction of the acidic-charged basement membrane substance was confirmed by 1) cytochemical positivity for periodic acid-Schiff, alcian blue (pH 2.5 or pH 1.0), colloidal iron, and periodic acid-methenamine silver, 2) immunocytochemical demonstration of laminin and type 4 collagen, and 3) ultrastructural identification of excessive formation of the basal lamina. Recognition of the "raspberry body" helps cytopathologists make a cytologic diagnosis of this chemotherapy-resistant malignancy disseminated in ascitic fluid.