The M30-monoclonal antibody recognizes a neo-epitope of cytokeratin 18 which is formed after caspase-cleavage during apoptosis. Caspase-cleaved cytokeratin 18 is released from apoptotic cells into circulation. The aim of this study was to evaluate the relationship between M30-antigen level and chemotherapy response in neoadjuvant treatment of breast cancer. Forty-two patients with invasive breast carcinoma received 4 cycles of anthracycline based neoadjuvant chemotherapy. Serum samples were obtained for assessment of M30-antigen levels before the administration of first chemotherapy cycle (baseline), and then after 24 and 48 hours for determination of chemotherapy induced apoptosis. M30-antigen levels at 24 and 48 hours were found to be significantly higher than baseline (p < 0.001, p = 0.003, respectively). M30-antigen levels in responders showed statistically significant increases at 24 and 48 hours (p < 0.001; p = 0.004, respectively), while statistically significant increases were not observed in nonresponders. Percentage change of M30-antigen levels was significantly higher in responders than nonresponders at 24 hours (p = 0.020). In conclusion, our study revealed a significant relationship between increases of M30-antigen levels in serum and overall response to therapy.