Protein S-100 immunoreactivity was observed in 5 of 50 breast carcinomas (3 infiltrating lobular and 2 infiltrating ductal carcinomas). A diffuse cytoplasmic staining was present in single cells and groups of cells. The majority of normal myoepithelial cells in ducts of unremarkable appearance next to tumor areas were stained in all 50 breast carcinomas. The 5 protein S-100 positive tumors all stained for prekeratin and 4 of them were vimentin-positive. No immunoreactivity for actin or NSE was observed in the 5 tumors. Electron microscopy did not distinguish the protein S-100 positive carcinomas from the 45 protein S-100 negative tumors. The significance of protein S-100 immunostaining in breast carcinomas is discussed.