Early detection of tumor-specific autoantibodies (auto-Abs) has the potential to be used for cancer screening and diagnosis. Whether auto-Ab may be useful to track metastatic progression or response to treatment is, however, largely unknown. To address these issues, the serological proteome was analyzed in an invasive but treatment-responsive mouse tumor model. Among 40 serum-reactive proteins identified by multiplex analysis, we chose to focus on glucose-regulated protein 78 (GRP78), a chaperone protein involved in the endoplasmic reticulum stress response. We first validated GRP78 as a protein overexpressed and mislocalized in tumor cells. We then documented that an increase in GRP78 auto-Ab titer preceded the detection of a palpable tumor mass, correlated with metastatic progression, and was influenced by the onset of tumor neovascularization. We also found that chemotherapy and radiotherapy, both leading to inhibition of tumor growth, oppositely influenced the anti-GRP78 immune response. Whereas radiation increased the concentration of GRP78 auto-Ab by three-fold, the auto-Ab titer was reduced in response to bolus or metronomic administration of cyclophosphamide. Finally, we established a decrease in auto-Ab-producing B lymphocytes in response to chemotherapy and the overexpression of GRP78 together with a strong immunoglobulin response in irradiated tumors. In conclusion, we identified GRP78 auto-Ab as an early marker of tumor and metastatic progressions. However, the multiple influences of anticancer treatments on the humoral immune system calls for caution when exploiting such auto-Ab as markers of the tumor response. © 2010 Neoplasia Press, Inc.