We used 2-color immunofluorescence flow cytometry (FACS 440) and monoclonal antibodies (Becton-Dickinson) to study peripheral blood lymphocytes subsets from 26 patients with well functioning renal allografts for longer than 5 years. Comparisons were made to a group of 25 healthy volunteers and 25 dialysis patients awaiting renal transplantation. As anticipated, there was no significant difference with respect to the absolute number or percentage of activated (T11+ and HLA-DR+) and cytotoxic (Leu-2+ and Leu-15-)T lymphocytes among the 3 groups. However, there was a significantly decreased absolute number (p equals 0.0001) and percentage (p equals 0.0001) of suppressor cells (Leu-2+ and Leu-15+) in the transplant patients compared to the healthy control group. No significant differences existed between the transplant and dialysis groups. There also was an increase in the percentage (p equals 0.000) but not in the absolute number of T helper lymphocytes (Leu-3+ and Leu-8-) in the transplant population compared to healthy controls. No significant difference existed between the transplant and dialysis groups. These findings suggest that a normal value of cytotoxic (Leu-2+ and Leu-15-) and activated (T11+ and HLA-DR+) T lymphocytes may be a good prognostic indicator of long-term survival of renal allografts. Also, the highly significant decrease in the number of Leu-2+ and Leu-15+ cells in long-term, well functioning allografts indicates that additional functional characterization of this subset may be necessary.