Previous research indicates that thymidine kinase I (TKI) possesses value as a tool for both prognosis and diagnosis in breast cancer. However, drawbacks to the existing radioassay for thymidine kinase have frustrated its clinical use. To overcome these drawbacks, we developed a monoclonal antibody to TK1. We have assessed this antibody for a linear antibody-antigen response and for reproducibility using ELISA techniques. We also have evaluated this antibody for TKI specificity as determined by Western blot. To test the accuracy of this monoclonal antibody further, we treated human MCF-7 breast cancer cells with tamoxifen and measured decreasing TKI activity and protein levels with the radioassay and with our monoclonal antibody in an ELISA, respectively. We then used the radioassay and our monoclonal antibody to measure TK1 activity and protein levels, respectively, in 218 serum samples of postoperative breast cancer patients and found a correlation between the two assays. Our results demonstrated that the TK1 immunoassay not only had a linear, reproducible, and specific response but accurately measured TK1 levels in both MCF-7 breast cancer cells and serum. Thus, our monoclonal antibody may demonstrate potential for practical use in a clinical setting for the management of breast cancer.