From a variety of Fc receptor-bearing cell/sensitized red blood cell combinations, mouse spleen cells, and sensitized SRBC were selected as an Fc-specific EA rosette assay system because only this mixture combined a high percentage (about 50%) of rosette-forming cells with complete absence of spontaneous rosettes and showed no influence of complement on the rosette formation. From studies on the minimal structural requirement of IgG both for mediation and inhibition of EA rosettes using IgG and several well-defined fragments, it appeared that both the CH2 and the CH3 domain of Fc are needed for optimal interaction with the lymphocyte Fc receptor. Finally, it was demonstrated that the assay system is able to detect "activated" Fc structures (here: heat-aggregated IgG) and to differentiate between varying amounts of such structures.