Fc gamma R are glycoproteins or phospholipoproteins respectively. Their molecular weight depends on type and origin of the Fc gamma R-coated cell and was determined between 40 and 115 KD. Fc gamma R can be estimated by means of rosetting assay or binding experiments with isotopic or fluorescein labeled immune complexes or aggregated IgG respectively. Depending on the cell type Fc gamma R are involved in different reactions after respective binding of their ligands. A lot of results are described in the literature, indicating that Fc gamma R bearing T lymphocytes are suppressor cells in the humoral immune response, but generally accepted data about the involvement of Fc gamma R in this suppression do not exist. Some models are represented in this paper.