Cellular receptors for the Fc fragment of IgG have been studied by the erythrocyte antibody (EA) rosette technique using either human red blood cells coated with anti-D IgG alloantibodies or pigeon red blood cells coated with aggregated IgG. Human lymphocytes were shown to form 4 to 35% of anti-D EA rosettes and 15% of E-aggregated IgG rosettes. Anti-D EA rosette values depended closely upon the amount and the source of anti-D antibodies, Ripley's type antisera giving the highest percentage of rosettes. Cell filtration through a nylon-wool column, a procedure known to remove B cells, caused a complete depletion of E-aggregated IgG rosettes and significant but only partial decrease of anti-D EA rosettes. A low but significant percentage of double rosettes were formed when anti-D-coated erythrocytes were mixed with aggregated IgG coated red blood cells or sheep red blood cells. Removal of EA-rosette-forming cells by passage on a Ficoll-Hypaque gradient K cell activity (assessed by lymphocyte-dependent antibody cytotoxicity using anti-HLA antibody-coated target cells) even when forming rosettes at low levels of erythrocyte sensitization with anti-D serum. Conversely, B cell markers (erythrocyte antibody complement rosettes or surface Ig) were only decreased at high levels of erythrocyte sensitization and remained unaffected after depletion of EA-rosette-forming cells formed at low levels of erythrocyte sensitization. These data suggest the existence of several populations of Fc receptor-bearing cells in human peripheral blood which may be differentiated according to the degree of erythrocyte IgG sensitization. Rosettes made with low concentrations of anti-D serum are mainly formed by non-B cells endowed with K cell activity, whereas those formed with high concentration of anti-D serum include B cells, monocytes, and K cells.