Immunohistological studies indicate that T cells and macrophages are the major components of human kidney allograft infiltrates. Recent work has demonstrated a division of T lymphocytes into 2 subpopulations with distinct functions on the basis of their expression of the CD45R antigen (CD45R+“naive” and CD45R−“memory” T cells). This study analyzes CD45R expression on circulating T cells and T cells infiltrating renal allo-grafts in patients undergoing rejection and/or cyclosporine nephrotoxicity. The percentage of circulating T cells that expressed CD45R in patients with rejecting (63±4) or stable grafts (66±3) was not different from values obtained for normal donors (62±3). In contrast, the percentage of T cells expressing CD45R infiltrating rejecting grafts was 21±2 and was not affected by the stage of rejection; in patients with CsA toxicity the value was 22±6. The reduced proportion of T cells that expressed CD45R in the allograft may reflect a change in status from the naive state due to alloantigenic stimulation (which can be demonstrated in vitro) and/or a propensity of memory T cells to enter or be retained in an allograft.