Immunohistochemistry using antibodies to cytokeratin 8 can serve as a valuable diagnostic tool for the differentiation of lobular from ductal carcinomas of the breast. In contrast with ductal carcinomas, which exhibit a peripheral-predominant immunostaining pattern, adjacent tumor cells "molding" to each other, lobular carcinomas exhibit a ring-like perinuclear immunostaining pattern, creating a "bag of marbles" appearance with neighboring tumor cells. This immunostaining pattern is stable even in the tumors that otherwise do not exhibit characteristic histomorphologic features (i.e., solid or pleomorphic type of a lobular carcinoma) and tumors that mimic growth patterns characteristic of the respective other tumor type (i.e., targetoid or single-file growth pattern in a ductal carcinoma). Furthermore, we demonstrate that ductal carcinomas express E-cadherin in a similar peripheral-predominant immunostaining pattern (33/33 cases), while all 15 lobular carcinomas were negative for E-cadherin, suggesting a role for E-cadherin in the architectural organization of the cytoskeletal scaffolding within the tumor cells.