The document summarizes the development and validation of a modified ADCC Reporter Bioassay to detect neutralizing antibodies against trastuzumab (Herceptin) in human plasma. The assay was optimized to use 10 ng/mL Herceptin and a 1:50 plasma dilution. Anti-trastuzumab antibodies inhibited the ADCC response in a dose-dependent manner, while isotype controls did not. The assay demonstrated good intra-assay precision and acceptable inter-assay precision. Cell passage number did not significantly impact assay results, indicating assay stability. In summary, the document describes modifying an existing bioassay to detect neutralizing antibodies against a biologic therapy.