Human lymphocytes with receptors for IgG (EA-RFC) were detected in peripheral blood by their ability to form rosettes with human erythrocytes sensitized with human (Ripley anti-CD) or rabbit IgG. EA-RFC were shown to be different from T-lymphocytes forming rosettes with sheep erythrocytes (E-RFC), both by direct studies of mixed rosette suspensions and by separation of EA-RFC and E-RFC by depletion of either EA-RFC or E-RFC by gradient centrifugation of the corresponding rosettes, as well as by fractionation of cells on HSA/anti-HSA columns, which very effectively and selectively retain EA-RFC. Proportions of EA-RFC were also within the normal range in patients with profound lack of T-cells and were not increased during in vitro lymphocyte culture in the presence of T-cell mitogens. The majority of EA-RFC was also distinct from B-lymphocytes as identified by membrane-bound Ig. Thus, EA-RFC could be separated from B-lymphocytes by fractionation on nylon columns retaining B-lymphocytes, but not EA-RFC, by fractionation on HSA/anti-HSA columns retaining EA-RFC but not B-lymphocytes, as well as by depletion of EA-RFC by gradient centrifugation of rosettes. The correlation between EA-RFC and B-lymphocytes in blood was also very poor, both in patients with pathologically low proportions of either normal or malignant B-lymphocytes as well as with abnormally high proportions. It is therefore concluded that EA-RFC represent a population of lymphocytes largely unrelated to human T- and B-lymphocytes as identified by conventional markers.