In this study a variety of cell-mediated immunity responses were performed in long-term (2 to 12 year) human leukocyte antigen-A (HLA) nonidentical renal transplant patients. All patients showed mixed lymphocyte culture (MLC) stimulation against donor, indicating a lack of tolerance. Of successful long-term transplant patients, 73 percent showed high serum levels of MLC-blocking activity. A consistent association of successful long-term transplantation and a specific defect in recipient ability to generate cytotoxic cells against donor was seen at the 8 to 12 year level. A close association of lymphocyte-dependent activity (LDA) and clinical chronic rejection was found, and LDA could be identified prior to the onset of clinical chronic rejection in most cases. Four patients had a positive LDA assay prior to transplant and all four went on to develop chronic rejection. Recipients taking immunosuppressive drugs within 24 hours of testing had a deficient capability to generate cytotoxic effector cells against indifferent individuals. These findings demonstrate a consistent association of in vitro cell-mediated immunity parameters and in vivo transplant function and may represent a valuable immunological monitoring system for long-term recipient follow-up.