T lymphocytes of 11 healthy humans were enriched and studied for Fc-IgG or Fc-IgM receptor expression by using a rosette technique, namely bovine red cells coated with rabbit IgG {EA-IgG} or IgM {EA-IgM}. Eighty percent (±9.2) of the T cells (mean ± S.D.) exhibited Fc-IgM receptors after culture (designated Tµ) and 22% (±9.4) expressed Fc-IgG receptors (designated Tγ), the latter occasionally decreasing after culture at 37°C. The two populations could be readily separated by interaction of Tγ cells with EA-IgG and Ficoll-Hypaque gradient separation. In the interface we found T non-γ cells, which consisted of over 90% Tµ cells and about 1% Tγ cells. In the pellet an enriched Tγ cell population was found, 79% (±8). After lysis of the bound EA-IgG, incubation of the cells at 37°C led to capping, endocytosis, and/or shedding of the Fc-IgG receptor. Culture of these cells led to expression of Fc-IgM receptors on these originally Tγ cells, whereas only a few cells re-expressed Fc-IgG receptors. The transition of Tγ → Tµ cells was also observed in T cells precultured for 24 hr and required immune complex (IC) interaction. Adding sodium azide or maintaining the cells at 4°C blocked Fc-IgM receptor expression. X-irradiation, cytochalasin B, and colchicine had no effect. Pronase treatment of positively selected Tγ cells uncovered Fc-IgG receptors, which did not cap, and after 40 hr in culture, T cells bearing Fc-IgG and Fc-IgM receptors were found. Therefore, Fc-IgG and Fc-IgM receptors do not appear to be markers for distinct T cell subsets under some conditions. Rather, a subset of T cells may conceivably express both classes of receptors at different functional stages. The interaction of Fc-IgG receptors with IC may serve to alter the overall expression of Fc-receptors.