Lymphocyte populations that bear Fc receptors for immunoglobulin G (IgG) (Fc gamma R) seem to be important in a variety of human diseases and have been studied extensively with several techniques to assess their status in human pathologic conditions. The immunologic role of these cells is quite important as they appear to contain the majority of T lymphocytes with suppressor activity and thus may have important regulatory functions. The Fc gamma R+ T lymphocytes are altered in several immunologic diseases and in some neoplastic conditions. We have studied FcR+ lymphocytes by a new method employing E-rosetting techniques in conjunction with a rosetting technique with chicken erythrocytes sensitized with rabbit antichicken erythrocyte IgG. In this fashion we have been able to define T lymphocytes and identify Fc gamma R+ lymphocytes in one simple step. This technique has been compared to other well-accepted methodologies and has given identical results. We have applied this technique to the examination of a group of women with nonmetastasized mammary carcinoma at different stages during their disease as compared with a group of normal, age-matched subjects. Our results indicate that Fc gamma R+ T lymphocytes detected by our mixed-rosette technique are significantly reduced in mammary carcinoma patients.