Six cases of sarcomatoid carcinoma of the breast (SCB) were studied with a panel of antibodies directed against epithelial and sarcomatoid components. The monoclonal antibodies (MoAb) AE-1/3, CAM 5.2, and CEA were used to detect epithelial differentiation; polyclonal antibodies against S-100 protein and MoAb against the intermediate filaments desmin and vimentin were used to detect mesenchymal differentiation in the sarcomatoid component. Six cases of invasive duct carcinoma (IDC) and two cases of cystosarcoma phyllodes (CP) were compared to SCB using the same panel of antibodies. In all three groups studied, the epithelial component in the majority of cases stained with anti-cytokeratin antibodies. S-100 protein antibodies stained the epithelial and sarcomatoid components in four cases of SCB; vimentin MoAb stained the epithelium in two cases and the sarcomatoid component in four cases of SCB, while MoAb CEA failed to stain any component of SCB. In contrast, the epithelium in five of six cases of IDC stained with CEA MoAb and only one of six stained for S-100 protein. Possible reasons for the discrepant immunohistochemical staining patterns among SCB, IDC and CP are discussed, in addition to the limitations and pitfalls of immunohistochemistry in diagnostic surgical pathology.