Undifferentiated nasopharyngeal carcinoma (NPC) is tightly associated with the Epstein-Barr virus (EBV) and very heavily infiltrated with T lymphocytes. We demonstrated recently that NPC epithelial cells produce immuno-regulatory molecules, including the Blast-2/CD23 antigen, which is induced in B lymphocytes upon infection by EBV. We demonstrate here that CD23 expression is a non-constant but highly specific feature of epithelial cells from NPC. The C15 and C17 NPC tumor cells express mainly the b form of CD23, which is known to be non-lineage-specific and IL-4-inducible. C17 cells were found also to weakly express the a form of CD23, which has been described as B cell-specific. In addition, several factors potentially released in vivo by tumor infiltrating lymphocytes (TILs) are able to regulate CD23 expression in NPC cells. In particular, we found that IL-4 was a potent inducer of CD23 expression in C15 cells, as shown at both the protein and the mRNA levels. These results, together with the already reported expression of class II MHC antigens and the release of IL-1 by NPC cells, suggest that the interactions between TILs and malignant cells are a key factor in NPC pathogenesis and development.