Epithelial cell adhesion molecule (EpCAM) is an attractive target for monoclonal antibody serotherapy because it is over-expressed in approximately 70% of epithelial cancers and their metastatic lesions. IGN101, the immunogenic formulation of the murine monoclonal anti-EpCAM antibody Mab17-1A, has been shown to evoke a strong humoral immune response in both monkey studies and early clinical trials. Notably, there was a reduction in the number of circulating EpCAM-positive tumor cells in the peripheral blood of treated cancer patients. In contrast to earlier publications by other groups, we could not detect an anti-EpCAM immune response upon treatment with Mab17-1A using a conventional but optimized anti-EpCAM ELISA. Therefore, in a novel experimental setup, sera of healthy immunized monkeys, normal human donors and cancer patients immunized with IGN101 were tested for reactivity against a series of overlapping synthetic peptides encompassing the entire sequence of EpCAM prepared by SPOT synthesis on cellular supports. Using this method, sera from normal donors reacted with different peptides compared to sera from healthy monkeys. However, the peptides were clustered in the same regions of EpCAM. Cancer patients generally had a lower reactivity to EpCAM peptides and immunization with IGN101 induced reactivity against a different set of peptides. Antibodies cross-reacting with both the IgG2a framework and with the Mab17-1A idiotype were identified. In summary, our data indicate that some EpCAM peptides may be recognized in a species-specific manner. At least seven EpCAM-derived peptides could be of diagnostic interest (QCQCTSVGAQ, ERVRTYWIII, ALQKEITTRY, TYWIIIELKH, IADVAYYFEK, AYYFEKDVKG, GQTLIYYVDE), while four out of these seven peptides may also possess therapeutic relevance (TYWIIIELKH, ALQKEITTRY, IADVAYYFEK, AYYFEKDVKG).