Normal and dysplastic mammary glands express immunocompetent S-100 protein positive dendritic cells (DCs), which are located in a regular pattern, in the suprabasal cell layer of the ducts and alveolar nodules. The epithelial cells, however, are S-100 protein negative. Since some breast cancers also express the S-100 protein, our aim was to check the diagnostic and prognostic value of the S-100 protein distribution combined with the tumor grade and expression of synaptophysin (Syn), chromogranin A (Chg A), c-erbB-2 oncoprotein and p53 protein in infiltrating and metastatic breast tumors. Applying immunohistochemical methods, we show in paraffin- or frozen breast tissue sections that in some cases of the infiltrating breast carcinomas, S-100 protein positive cells do not appear, whereas in other cases, either S-100 protein positive DCs are closely associated with cancer cells, or the cancer cells themselves stain positive to S-100 protein. However, we found no correlation between the S-100 protein expression and other investigated parameters.