An in vitro line was derived from an American Burkitt lymphoma, designated Ra No. 1, which produced malignant tumors when inoculated into thymus-deficient nude mice. The cells have B-lymphocyte characteristics, with surface-associated mu and kappa chains and Epstein-Barr virus (EBV) receptors, and can be readily infected with EBV in vitro. B95-8 virus induced EBV-determined nuclear antigen (EBNA) but not early antigen (EA) in Ra No. 1 cells, whereas P3HR-1 virus induced both EBNA and EA, but the EA level was much lower than in the prototype Raji strain, EBNA and EA levels were comparable in Ra No. 1 and in the previously established, EBV-infection-sensitive BJA-B lymphoma. The two lines differed, however, because BJA-B could be converted into a permanent EBV-carrying line by the B95-8 but not by the P3HR-1 virus strain, whereas Ra No. 1 could be converted by the P3HR-1 virus. This demonstrates that different B-lymphocyte-derived lymphoma lines can vary with regard to the restrictive control they can exert on the same virus strain. Furthermore, virus strains vary in their sensitivity to the restrictions of the same host cell. Permanent EBV conversion of the Ra No. 1 cell did not appear to change the cellular controls, as judged by the unchanged sensitivity of the cell to viral antigen (EA, viral capsid antigen) induction by P3HR-1 virus superinfection.