Sera from juvenile diabetic and nondiabetic controls were tested, using the immunoperoxidase method, for the presence of islet binding immunoglobulins. All diabetic sera tested contained an islet binding protein and on the average the sera were more positive than age matched controls. Normal adult sera are undifferentiated from juvenile diabetic sera. Most sera from children less than two years of age did not bind to islet tissue and sera from cystic fibrosis patients had a markedly diminished ability to bind to islet tissue. The binding protein appears to be an immunoglobulin which selectively reacts with elements of the islet beta cell.