The blastogenic response of T-cell populations to infectious EBV (B95-8 strain) was studied. The addition of virus increased the [3H]thymidine uptake of unfractionated lymphocytes from seropositive and seronegative subjects, and from cord blood, but not from patients with infectious mononucleosis. The same amount of virus evoked a small response in purified T cells from seropositive donors, but not in T cells from the other sources. The addition of autologous B lymphocytes with EBV adsorbed on their surfaces caused a more significant blastogenic response, which was even more pronounced in the presence of mercaptoethanol.