Basement membrane status in well-differentiated and undifferentiated carcinomas of the ovary was examined and compared immunohistochemically by using antibodies which react with type IV collagen and laminin. In well-differentiated carcinomas, which included 8 serous, 8 mucinous, 4 endometrioid and 4 clear cell adenocarcinomas, the staining showed a regular and linear pattern in the interface between epithelium and stroma. However, in 8 cases of undifferentiated carcinoma, the staining for two antibodies was irregular, being either completely negative or focally positive in the portion surrounding the carcinoma cell nests. In some cases, intracytoplasmic staining was observed. Staining patterns for two antibodies were somewhat different within undifferentiated carcinoma cases. It is concluded that in ovarian undifferentiated carcinomas, the architecture of basement membrane is substantially altered compared to their differentiated counterparts.