Twenty-eight undifferentiated carcinomas (UCs) were immunohistochemically investigated with antibodies against cytokeratins (CKs), vimentin, p53 protein, c-erbB-2 protein and CEA. The diagnoses were based on the findings of conventional histopathology, immunohistochemistry and electron microscopy. CKs8, 18 and 19 were the CKs most frequently present in these tumors, in 61%, 61% and 82% of the cases, respectively. Nine of the 28 (32%) UCs were CKs5/6 positive. Expression of CK20 was found in three (11%) cases. Four UCs were sub-type Cks negative, but one of them was confirmed AE1/AE3 positive. P53 protein overexpression was found in nine (32%) cases. One of the patients with p53 protein positive tumors has been alive for 174 months. Nine (32%) UCs expressed vimentin, which included all of the three thyroid UCs. Comparing with our previous study of squamous cell carcinomas, we found that vimentin and CK18 are more frequently expressed in UCs. The overexpression of p53 protein is similar in the two groups of carcinomas and thus, p53 protein is not a differentiation marker in these tumors. Finally, we recommend the use of a CK "cocktail of antibodies" in the diagnosis of UCs.