Mouse monoclonal antibodies E29 and 115D8 have been raised in the human milk fat globule membrane. The distribution of the antigen detected by these two monoclonal antibodies has been examined in formalin-fixed paraffin-embeded human ovarian tumors by an indirect immunoperoxidase technique. The highest positive rate of E29 and 115D8 was obtained in the malignant group suggesting that there is relationship between this positive rate and the histological malignancy. As for serous cystadenocarcinomas, a strong staining area of E29 was observed in the diffuse cytoplasm in poorly differentiated cases. These results suggest that the presence of E29 is a useful tumor marker for ovarian cancer.