We studied the effects of a hydroxyl radical scavenger, dimethylthiourea (DMTU), on syngeneic islet grafts. Six hundred fresh islets taken from two Wistar rats were transplanted intraportally into other Wistar rats made diabetic with streptozotocin. DMTU was given to five recipients intraperitoneally for a month while nine controls received only intraperitoneal saline. The DMTU-treated group had significantly lower fasting plasma glucose levels at 1.5, 4.5, 5, 5.5, 6, 7, 9 and 11 weeks and had higher mean fasting body weights between the 2.5th week and their eventual sacrifice. Their islets also survived significantly longer than did those of the controls (73.6 +/- 3.4 vs. 21.6 +/- 9.4 days). This suggests that oxygen free radical production endangers graft survival and that the hydroxyl radical scavenger DMTU protects syngeneic islet grafts.