Immunological monitoring of peripheral blood cells of renal transplant patients may be helpful for diagnostic and prognostic purposes. We therefore enumerated the percentage of Leu2a+ cells as well as the occurrence of HLA-DR activation markers within this population. Since Leu2a+Leu15+ lymphocytes display suppressor functions in-vitro, this population was included in the study. Blood samples of 50 renal transplant patients within 3 months of transplantation and 41 pretransplant samples were investigated. As controls, peripheral blood cells from 10 healthy volunteers and 20 renal transplant patients with stable graft function more than 6 months after transplantation were examined. Just prior to transplantation the percentages of Leu2a+ cells, as well as the percentages of Leu15+ and DR+ cells within this population, varied considerably. Shortly after transplantation, clinical rejection episodes and cytomegalovirus (CMV) infections correlated with decreased percentages of Leu2a+ cells (P = 0.024 and 0.016, respectively). In nine patients with a decreased percentage of Leu2a+ cells shortly after transplantation but no rejection, the percentage of Leu2a+ cells normalized within 14 days. No correlation was found between increased percentages of Leu15+ or DR+ cells and rejection episodes, although the percentages of Leu2a+Leu15+ and Leu2a+DR+ cells increased significantly shortly before rejection (P less than 0.005). A decreased percentage of Leu2a+ cells, together with increased percentages of Leu15+ and DR+ cells, was seen in association with CMV infections (P less than 0.005). From this study we conclude that low percentages of Leu2a+ cells correlate with rejection episodes or CMV infections, whereas low percentages together with increased percentages of Leu2a+DR+ and Leu2a+Leu15+ cells are associated with CMV infections, especially in patients treated with high doses of cyclosporine.