Twenty cases of infiltrating ductal breast carcinomas were stained wit
h the mouse monoclonal antibody H, an IgM, in order to estimate the di
stribution of the epitope (a carbohydrate moiety) recognized by this a
ntibody in the malignant cells. The results showed that in all cases e
xamined, the epitope was present in the cytoplasm of the neoplastic ce
lls arranged mainly in a diffuse pattern. To identify the polypeptides
that carry this epitope, we used extracts from the MCF-7 human breast
carcinoma eel line in immunoblotting experiments. In addition to cyto
keratin 8 this epitope was also found on five polypeptides originating
from Triton X-100-soluble (M(r)x10(-3) of 232, 67, and 37) and from t
he Triton X-100-insoluble (M(r)x10(-3) of 51, and 50) fractions, respe
ctively. The contribution, if any, of these polypeptides to the staini
ng patterns is not known. These data reveal the existence of a common
epitope between cytokeratin 8 and other cellular polypeptides. They al
so serve as a note of caution in the interpretation of immunohistochem
ical staining when anti-cytokeratin antibodies are used.