A time-lapse study of granulocyte cytotoxicity was performed with sera from 13 renal allograft recipients (living related donor; 10, cadaver donor; 3) using a droplet microgranulocytecytotoxicity test. Of eight patients who had previously received blood transfusions, two gave positive reactions for the granulocyte cross match test and developed acute rejection after renal transplantation. Of seven patients with rejection of a renal graft, four were positive for granulocyte cytotoxicity before and after operation. Five patients showed good results for renal graft and four of these were negative for granulocyte cytotoxicity both before and after operation. Some of the patients with good grafts gave repeated positive reactions for granulocyte cytotoxicity but later showed poor grafting. Results for granulocyte cytotoxicity which were negative early tended to become positive about 10 days before acute rejection. Granulocyte cytotoxicity did not correlate well with HLA-matching or white blood cell counts. Lymphocyte cytotoxicity examined simultaneously was negative in the sera of all 13 patients.