The positive binding activity of lectin, peanut agglutinin (PNA), against the mucinous malignancies of the ovary was studied in order to clarify biologic differences among those lesions using immunoperoxidase method (ABC). A total of 23 cases were included in this study and they were classified as 10 cases of mucinous cystadenocarcinoma, 9 mucinous tumors of borderline malignancy and 4 pseudomyxoma peritonei, histologically. Nine of 10 cystadenocarcinomas and all cases of pseuomyxoma peritonei showed more than moderate degree of positive binding activity (>2+) with PNA in the neoplastic epithelial cells. In the cases of borderline malignancy, only 3 of 9 revealed as much similar binding pattern with PNA as cystadenocarcinoma group, in contrast, minimal degree of positivity (1+) was noted in the remainder. These findings may suggest heterogeneity in the biochemical characteristics among the cases of borderline lesion. And it is proposed that the higher PNA binding cases in ovarian mucinous borderline malignancy require extensive sampling by multiple sections and further careful follow-up study.