T-cell suspensions enriched and depleted for the subpopulation with FcR for antigenbound IgG (EA-RFC) have been tested for the responding capacity in MLC/CML. The results reveal that the EA-RFC depleted T cell suspension (enriched for FcR-ve T cells) have a greater proliferative capacity but unchanged cytotoxicity compared to the unfractionated T cell suspensions. On the contrary, the EA-RFC enriched T cell suspensions (enriched for FcR + ve T cells) have only weak proliferative capacity and cytotoxicity. These results indicate that the major part of proliferating T-amplifiers as well as cytotoxic T-precursor cells are FcR-ve. Fractionation of cytotoxic effector cells day six of culture reveal that cytotoxic cells are found among EA-RFC depleted as well as enriched suspensions. The cytotoxic capacity of effector cells was found to be substantially reduced after EA rosette formation due to a blocking and sterical inhibitory influence of the EA complexes on the close effector-target contact necessary for cytotoxicity. These results indicate predominance of FcR + ve cytotoxic T cells. The rosette indicator system used here for identification and definition of FcR cannot separate cytotoxic T-precursor cells and strongly proliferating T-amplifiers found to be FcR-ve. On the contrary, cytotoxic T-effector cells are predominantly FcR + ve.