Preimmunization which occurs as a result of transfusion and pregnancy is postulated to be responsible for most early kidney transplant failures. A cell-mediated cytotoxicity test was evaluated as an assay of sensitization. Statistically significant higher levels of immunization could be shown in 29 patients who were in the process of rejecting, or who had already rejected, a transplant when compared to 23 patients with functioning kidney transplants, 53 patients on hemodialysis waiting for transplants, and 23 normal persons. The ability of lymphocytes to kill allogeneic target lines in culture may therefore be a method by which presensitization can be detected before transplantation and by which the state of immunity can be monitored after transplantation.