Known numbers of thymus-dependent (T) lymphocytes, obtained by positive selection from the blood of acute infectious mononucleosis (IM) patients and from control donors, were added to target cultures of foetal mononuclear cells within 0-7 days of exposure of the target cells to one of a range of doses of Epstein-Barr (EB) virus. The subsequent outgrowth of virus-transformed foetal cells was markedly inhibited by the presence in the cultures of IM-derived T cells, whilst similar numbers of T cells prepared either from cord blood or from adult donors seronegative for EB virus had little or no inhibitory effect. Target foetal cells treated with papain to remove any viral envelope material remaining on the cell surface after infection, were just as sensitive as untreated cells to the addition of IM-derived T cells. It is concluded that the inhibition cannot be mediated through recognition either of viral envelope structures on the surface of infected cells or of the antigenically related virus-determined membrane antigen, MA, but must depend upon recognition of the lymphocyte-detected membrane antigen, LYDMA. The regularity with which IM-derived T cells block the outgrowth of virus-transformed foetal cells suggests that LYDMA consistently appears on the surface of infected foetal cells before the establishment of transformed foci, but is unlikely to be directly associated with the cells' existing histocompatibility antigens.