The immunoperoxidase localization of epithelial antigen (keratin, epithelial membrane antigen) was identified in 18 cases of sarcomatoid carcinomas localized in the upper aero-digestive tract. These tumors are an association of a differentiated squamous cell carcinomatous component and of spindle, or pleomorphic cells component. The anti-keratin monoclonal antibody KL1 stained the sarcomatoid component in 4 cases. A continuous staining between differentiated squamous cells and spindle cells in observed in two cases. The identification of cytokeratin is better with KL1 than with the polyclonal anti-keratin antiserum KT. The monoclonal antibody HMFG2 is not retained. These results suggest the dedifferentiation of epithelial cells in sarcomatoid component. They confirm the heterogeneity of these tumors, which has been previously found in ultrastructural studies. These immunostaining are helpful in recognizing sarcomatoid carcinoma, when they are only represented by the sarcomatoid component.