Immunohistochemical studies using antisera to keratin and secretory component (SC) and monoclonal antibody against leukocyte common antigen (LCA) were performed on 92 biopsy and autopsy specimens taken from 65 patients with nasopharyngeal carcinoma. Five biopsy specimens from malignant lymphoma of the nasopharynx and tonsil, and 20 biopsy specimens of nasopharyngeal epithelium were also included in the study. Keratin was positively stained in all squamous cell carcinomas and nonkeratinizing carcinomas, and 38 of 46 undifferentiated carcinomas (82.6%), but in no malignant lymphomas. LCA was intensely stained in all malignant lymphoma, but in no nasopharyngeal carcinomas. SC was positively stained in two of the 46 undifferentiated carcinomas (4.3%). Nasopharyngeal carcinoma is a definite malignant epithelial neoplasm and can be distinguished from malignant lymphoma by immunostaining for keratin and LCA. Some undifferentiated carcinoma may show cellular differentiation toward ciliated epithelium.