There are limited data that compare the usefulness of mammaglobin with gross cystic disease fluid protein-15 (GCDFP-15) in the identification of breast carcinomas. Whole tissue sections of 29 breast carcinomas with matched lymph node metastases and 63 breast carcinomas on tissue microarray were stained with mammaglobin cocktail and GCDFP-15 antibodies. In addition, tissue microarrays (US Biomax, Rockville, MD) containing 544 different human tumors were also stained with the mammaglobin antibody cocktail. Positive staining was seen in 67 (55.4%) of 121 breast carcinomas with mammaglobin and in 28 cases (23.1%) with GCDFP-15. In the majority of cases, the staining intensity and number of cells staining were higher with mammaglobin than with GCDFP-15. Positive mammaglobin staining was also seen in 44 (8.1%) of 544 nonbreast tumors. Mammaglobin is a more sensitive marker than GCDFP-15 for breast carcinoma; however, it lacks the specificity of GCDFP-15.