Binding to the antigen defined by the monoclonal antibody HMFG2 was demonstrated in more than 94% of human epithelial ovarian carcinomas by the indirect immunoperoxidase technique. The extent of the expression of this antigen related to tumor differentiation; well-differentiated tumors expressed more antigen than poorly differentiated tumors. Marked variation of tumor grade and antigen expression was demonstrated in individual tumors and this may be a limiting factor in the use of radioisotope linked HMFG2 for tumor therapy. A similar prevalence was found in the expression of the antigens defined by the monoclonal antibodies HMFG1 and F36/22; whereas the AUA1 antigen was demonstrated in 68% of cases. The patterns of expression of the HMFG1, HMFG2, and F36/22 antigens suggest that they may be related antigens.