The monoclonal antibody Ki-67 is directed against a human nuclear antigen occurring in proliferating cells. As a rule it is absent in quiescent cells. Seven cases of squamous cell carcinomas with different pathohistological features are investigated by immunostaining. The antibody Ki-67 is suitable for use as a marker for proliferative compartments of head and neck tumors. Squamous cell carcinomas show a correlation between Ki-67 labeling index and histological differentiation. In highly differentiated parts of the carcinomas, an additional granular immunostaining in the entire cytoplasm of some squamous epithelial cells was observed.