SUMMARY Immunological surveillance of 10 human renal allograft recipients was undertaken through serial study of lymphoblastoid cells in their peripheral blood. These cells were identified by their spontaneous uptake of 3H-thymidine. Two methods were used. In the first, 108 lymphocytes were cultured in 3 ml of media and plasma. In the second, 0.05 ml of blood was cultured in 2 ml of culture fluid. Results obtained with both methods allowed for the recognition and prediction of impending allograft rejection crises and for judging the adequacy of immunosuppressive therapy for these crises. The micromethod correlated with the method using 106 cells (r=0.82, significant at 0.001 level). Monitoring of lymphocyte responsiveness to phytohemagglutinin was of no value in evaluating efficacy of immunosuppression.