The expression of HLA-DR by T lymphocyte subpopulations recognized by monoclonal antibodies and flow cytometry was monitored in 10 normal controls, 15 patients on hemodialysis, 25 recipients of a renal allograft with stable graft function, 16 transplant recipients suffering rejection episodes, and 4 transplant recipients with cytomegalovirus (CMV)4 infection. No significant differences in HLA-DR expression were observed in the OKT4-reactive subpopulation among these controls and patients. In the 16 patients who suffered rejection episodes a significant increase in HLA-DR expression by OKT8-reactive cells was observed. In the 10 patients in whom rejection episodes occurred later than 1 week after transplantation, the percentage of OKT8-reactive