The expression of Ia-antigen on four different Epstein-Barr virus associated nasopharyngeal carcinomas xenografted into athymic mice could be detected by the monoclonal antibody OKIa. Xenografts of four additional head and neck tumors other than nasopharyngeal carcinoma and one xenograft of a metastatic melanoma cell line were negative for the Ia-antigen. Control antibodies OKT3, OKT4, OKT9, OKM1 and Leu7 were negative with all nasopharyngeal carcinomas and the non-nasopharyngeal carcinoma xenografts. Complement receptors as the presumed receptors for the Epstein-Barr virus could not be detected on xenografted nasopharyngeal carcinoma cells but were found on freshly prepared peripheral blood lymphocytes as well as on the Epstein-Barr virus transformed lymphoblastoid cell line QIMR-WIL. The possible role of the Ia-antigen on nasopharyngeal carcinoma cells in respect to the Epstein-Barr virus association of this malignancy is discussed.