In this study we have measured glycosylated haemoglobin in the blood of 22 rats made diabetic with streptozotocin. This significantly increased from a normal level of 1.6 +/- 0.2% to reach a maximum, plateau level, of 6.6 +/- 0.3% at 45 days. At this time, 16 of these animals were given intra-portal transplants of purified isologous pancreatic islets. By varying the number of islets transplanted, animals were produced with normal blood glucose levels on random sampling, but with a spectrum of glucose control as measured by the glucose tolerance test. The glycosylated haemoglobin level was also measured at 30, 60 and 120 days post transplantation. This was significantly reduced (in some cases back to normal) and at 60 and 120 days there was a significant correlation (p less than 0.001) between glycosylated haemoglobin and the glucose clearance (K value). These results suggest that the measurement of glycosylated haemoglobin can be used as a simple and accurate index of glucose metabolism in islet transplanted rats.