Complement receptor 2 (CR2, CD21), the receptor for both the C3d,g portion of human complement component C3 and the Epstein-Barr virus, has been recently described on peripheral T cells. By using dual stain flow cytometric analysis, we have also observed that a peripheral T lymphocyte subpopulation of normal healthy donors bears CR2 in a range varying from 1.1 to 23.2% (mean 12.6%) of total CD3+ cells. T lymphocytes from nine patients with inactive SLE expressed CR2 in a similar range. Three patients with active SLE were also studied. One of them, having neuropathy and glomerulonephritis, displayed an expansion of the CR2 T cell subpopulation which reached as much as 89% of total CD3+ cells. To examine potential functional roles of T cell CR2, cells from a Jurkat-derived CR2 expressing T cell line were found to bind in vitro to human CR2-, complement-coated K562 cell targets in a CR2- and complement-dependent fashion. Based on these studies, we hypothesize that CR2 might act to increase adherence of T cells to nucleated target cells bearing C3d,g, a function which may be relevant to cytotoxicity or other T cell activities requiring cell-cell interaction.