The GC-BJAB cell line, which carries the Epstein--Barr virus (EBV), was derived from an EBV-genome-negative lymphoma line (BJAB) by EBV infection in vitro [G. B. Clements, G. Klein, and S. Povey (1975) Int. J. Cancer, in press]. Both lines grow at a similar rate at 37 degrees but they differ at other temperatures. BJAB grows well at 34 degrees, 37 degrees, and 39 degrees. GC-BJAB grows at 37 degrees and 39 degrees, but grows poorly at 34 degrees. At 37 degrees, GC-BJAB cultures can be maintained at the viable state for a long time after having reached saturation density (approximately 10(6) cells per ml). In contrast, BJAB cultures die very soon after having attained similar maximum density. Since the identity of the two cell lines has been critically established [Clements et al.; E. Svedmyr and M. Jondal (1975) Proc. Nat. Acad. Sci USA 72, 1622--1626; G. Klein, to be published; J. Zeuthen, personal communication] the remarkable differences in their growth properties must be attributed to the EBV genome.