The leucocyte migration inhibition test has been studied in a series of 192 renal allograft recipients. Seventy-seven patients showed no evidence of inhibition in the early post-transplant course, but 31 of these demonstrated clinical evidence of rejection, a false-negative rate of 16%. The remaining 115 recipients all demonstrated inhibition, with 13 of these showing no clinical evidence of rejection, a false-positive rate of 6.7%. Early antirejection therapy on the basis of inhibition did not result in improved kidney survival when compared with those recipients who did not receive specific therapy until there was clinical evidence of rejection. The leucocyte migration inhibition test did not detect changes attributable to humoral factors, which probably accounts for the high false-negative rate, and has not proved to be sufficiently reliable to be of value clinically as a single test. A combination of tests designed to detect both humoral and cellular factors responsible for rejection deserves further study.