Morphologic differentiation of breast carcinoma from nonmammary malignancies in fluid specimens can be a diagnostic challenge. Immunocytochemistry is often employed in the differential diagnosis. In this study, we evaluated the expression of mammoglobin (MGB1) in body-cavity fluid specimens and compared its efficacy as a marker for metastatic breast carcinomas with that of gross cystic disease fluid protein-15 (GCDFP-15). Cell blocks from 40 fluid specimens were immunostained with monoclonal antibodies against MGB1 and GCDFP-15. They included 15 breast carcinomas and 25 nonmammary carcinomas (10 lungs, 10 ovaries, 3 gastrointestinal tracts, 1 kidney, and 1 urinary bladder). Positivity was defined as the presence of cytoplasmic staining in 10% or more carcinoma cells. Thirteen (87%) and seven (47%) breast carcinomas showed positive staining with MGB1 and GCDFP-15, respectively. Three (12%) nonmammary carcinomas (2 ovarian and 1 colonic) showed positive MGB1 staining; one (3%) nonmammary carcinoma demonstrated positive GCDFP-15 staining. The differences of MGB1 and GCDFP-15 staining between breast and nonmammary carcinomas were statistically significant (P < 0.05). Both MGB1 and GCDFP-15 are specific markers for metastatic breast carcinomas in cell block fluid specimens (88 vs. 96%). However, MGB1 is more sensitive than GCDFP-15 as a marker for metastatic breast carcinoma (87 vs. 46%). © 2009 Wiley-Liss, Inc.