Treatment of overtly diabetic NOD mice with anti-lymphocyte serum (ALS), a polyclonal anti–T-cell antibody, abrogates autoimmunity and achieves partial clinical remission. Here we investigated whether the addition of exendin-4, a hormone that stimulates insulin secretion and β-cell replication and differentiation, improves induction of remission by ALS. Transient treatment of overtly diabetic NOD mice with ALS and exendin-4 achieved complete remission in 23 of 26 mice (88%) within 75 days, accompanied by progressive normalization of glucose tolerance, improved islet histology, increased insulin content in the pancreas, and insulin release in response to a glucose challenge. Syngeneic islets transplanted into mice cured by treatment with ALS plus exendin-4 remained intact, and cotransfer of lymphocytes from cured mice delayed diabetes induction by adoptive transfer, suggesting the long-lasting presence of autoimmune regulatory cells. Although ALS alone also achieved reversal of diabetes, the frequency of remission was low (40%). No treatment or exendin-4 alone failed to produce remission. These results show that exendin-4 synergistically augments the remission-inducing effect of ALS. The addition of β-cell growth factors, such as exendin-4, to immunotherapy protocols with anti–T-cell antibodies presents a potential novel approach to the cure of patients with new-onset type 1 diabetes.