The antibody-dependent cell-mediated cytotoxicity (ADCC) effector activity and phytohemagglutinin (PHA) response of kidney transplant patients were monitored by a new whole blood microassay that permitted performance of 20 tests using only 1 ml of blood. Eleven patients were followed on as many as 55 different occasions with these tests. The ADCC activity of dialysis patients was essentially the same (78% chromium release as compared with 83% in normal controls). However, patients with kidney transplants undergoing immunosuppression had approximately one-half the chromium release (40%). Daily fluctuation in the chromium release could not be used as a predictor of subsequent rejection. The PHA response was as much as 50-fold less in the immunosuppressed patients. Both values also tended to be slightly higher among patients who were doing well than those who were doing poorly. This may reflect the need for greater immunosuppression in the rejecting patients.