Epstein-Barr virus (EBV)-infected cells may sustain three distinct forms of virus latency. In lymphoblastoid cell lines, six EBV-encoded nuclear antigens (EBNA1, 2, 3A, 3B, 3C, -LP), three latent membrane proteins (LMP1, 2A, 2B), and two nuclear RNAs (EBERs) are expressed. This form of latency, termed latency III, is also encountered in some posttransplant lymphoproliferative disorders. In EBV-positive cases of Hodgkin's disease, the EBERs, EBNA1, and the LMPs are expressed (latency II), whereas in Burkitt's lymphoma (BL) only the EBERs and EBNA1 have been detected (latency I). We have studied the expression of EBV proteins in 17 cases of EBV-positive endemic BL by immunohistology. Expression of LMP1 was seen in variable proportions of tumor cells in two cases and EBNA2 was detected in some tumor cells in three other cases. Also, the BZLF1 trans-activator protein was expressed in a few tumor cells in 6 cases, indicating entry into the lytic cycle. A phenotypic drift from latency I to latency III has been observed previously in some BL cell lines. Our results suggest that a similar phenomenon may occur in BL in vivo and indicate that the operational definition of EBV latencies is not easily applied to human tumors.