MUC16 is a well-validated cell surface marker for serous adenocarcinomas of the ovary and other gynecologic malignancies that is distinguished by highly repetitive sequences (“mucin repeats”) in the extracellular domain (ECD). We produced and compared two monoclonal antibodies: one (11D10) recognizing a unique, nonrepeating epitope in the ECD and another (3A5) that recognizes the repeats and binds multiple sites on each MUC16 protein. 3A5 conjugated to cytotoxic drugs exhibited superior toxicity against tumor cells in vitro and in tumor xenograft models compared with antibody-drug conjugates of 11D10. Importantly, drug conjugates of 3A5 were well tolerated in primates at levels in excess of therapeutic doses. Additionally, the presence of circulating CA125 in a rat model did not exacerbate the toxicity of 3A5 drug conjugates. We conclude that targeting the repeat MUC16 domains, thereby increasing cell-associated levels of drug-conjugated antibody, provides superior efficacy in vitro and in vivo without compromising safety. [Cancer Res 2007;67(10):4924–32]