Epstein-Barr (EB) virus-immortalized B lymphocytes coexpress the EB viral latent gene products (EB viral nuclear antigens 1 to 6, the latent membrane protein and the terminal protein gene products) and the cellular activation antigen CD23. Immortalized B cells can be separated from those which are infected but not immortalized on the basis of CD23 expression as early as 2 days after in vitro infection. In the present report we have confirmed these data, but show that if left in culture for 7 days after infection before separation the CD23-negative cells show a donor-related ability to become CD23-positive and immortalize. CD23-negative cells separated 2 days after infection can be induced to immortalize by the addition of low Mr B cell growth factor but not by the addition of recombinant interleukin 1, 4 or soluble CD23. At 2 to 3 days after infection the EB viral nuclear antigens 1, 2 and the high Mr species 3, 4 and 6, as well as the latent membrane protein can be detected in the CD23-positive fraction. In contrast at this time only nuclear antigens 1 and 2 could be detected in the CD23-negative fraction. This difference in gene expression may account for the inability of the CD23-negative fraction to immortalize. In the light of these observations the mechanism of viral persistence in vivo is discussed.