Epstein-Barr virus (EBV) has potent cell-growth-transforming activity for human B lymphocytes in vitro, yet appears to persist in the circulating B-cell pool of virus carriers in vivo as a largely asymptomatic (i.e., non-growth-transforming) infection. The true nature of this infection, and the identity of the cells involved, remain to be determined. Studies of Lewin et al. (1987) have suggested (i) that the frequency of virus-infected cells in the circulating B-cell pool differs in different buoyant density fractions, being most abundant in the low-density population, and (ii) that rare virus-infected cells with the capacity for direct in vitro outgrowth to EBV-transformed cell lines are segregated within the high-density population. We have repeated this work using B-cell fractions from a much larger panel of asymptomatic virus carriers and find (i) that the incidence of virus-infected B cells is not significantly different between high- and low-density fractions, and (ii) that virus-infected cells from both fractions give rise to EBV-transformed cell lines in culture predominantly through a 2-step mechanism of virus replication and secondary infection rather than by direct outgrowth.