Epstein-Barr virus (EBV) related specific killing was demonstrated previously in the T cell enriched subpopulations of blood lymphocytes of IM patients. In the present work we demonstrate that the cytotoxic cells belong to the Fc negative T subset. T cells in the blood of IM patients can be divided in 2 categories, 1 Fc positive with non discriminative cytotoxicity and the other, Fc negative with selective cytotoxicity against EBV genome carrying lymphoblastoid cell lines.