GATA-3 is a transcription factor that has recently been identified by immunohistochemistry to be highly expressed in urothelial and breast carcinomas (CAs). We sought to determine the potential utility of GATA-3 in identifying metastatic breast CA, and to compare its utility with the standard breast markers, GCDFP-15, and mammaglobin A. We identified an archival series of 338 formalin-fixed paraffin-embedded whole-tissue sections of various CAs. Using standard immunohistochemical (IHC) techniques we used mouse monoclonal antibodies to GATA-3 (clones L50-823, HG3-31), GCDFP-15 (23A3), and mammaglobin A (31A5). Both clones of GATA-3 showed positivity in 96% of non–triple-negative breast carcinomas (TNBCs), L50-823 and HG3-31, demonstrating expression in 87% and 63% of TNBCs, respectively; GCDFP-15 and mammaglobin A were expressed in 69% and 61% of non-TNBCs, respectively, and 10% and 17%, of TNBCs, respectively. The L50-823 clone manifested a lower specificity in identifying breast CAs (84%) than did the HG3-31 clone (97%). Both monoclonal antibodies to GATA-3 are very sensitive reagents for the identification of breast CA, surpassing antibodies to GCDFP-15 and mammaglobin A, and offer a significant improvement in identifying TNBCs. However, the L50-823 clone showed a lower level of specificity, which may qualify its utility in the setting of CAs of unknown primary.