CA-549 is a high molecular weight acidic glycoprotein found in the serum of breast cancer patients. Detection of CA-549 in serum using an immunoradiometric assay has [1] correlated with disease course in breast cancer patients and [2] aided in establishing the diagnosis of breast cancer in patients with metastatic disease. This study examines the expression of CA-549 using immunohistochemistry in normal breast, benign breast disease, breast carcinoma, and a variety of other carcinomas. In addition, in 29 patients both serum and tissue samples were available for correlation. CA-549 was constitutively expressed in normal breast tissue, but immunohistochemical positivity was restricted either to the luminal aspect or entire cell membrane. All patients with benign breast disease had normal levels of CA-549 despite immunohistochemical positivity. Nearly all (98 percent) of breast carcinomas showed reactivity for CA-549, with a majority (82 percent) of the cases showing cytoplasmic positivity. In patients with both serum and tissue studied, those with cytoplasmic staining of the breast carcinomas had mean serum level of 174 U per ml (range 1.9 to 785), compared to 37.3 U per ml (range 2.1 to 86.8) in those with only membrane or luminal staining of tumor (p = 0.0578). Immunoreactive CA-549 was found in many normal epithelia and in other types of carcinomas. CA-549 is [1] constitutively expressed on the cell membrane of normal breast epithelium, [2] commonly present in the cytoplasm of breast carcinomas, and [3] found often in a variety of carcinomas.