Type 1 diabetes mellitus results from the autoimmune destruction of the β cells of the pancreatic islets of Langerhans and is recapitulated in the nonobese diabetic strain of mice. In an attempt to rescue islet loss, diabetic mice were made normoglycemic by islet transplantation and immunization with Freund's complete adjuvant along with multiple injections of allogeneic male splenocytes. This treatment allowed for survival of transplanted islets and recovery of endogenous β cell function in a proportion of mice, but with no evidence for allogeneic splenocyte–derived differentiation of new islet β cells. Control of the autoimmune disease at a crucial time in diabetogenesis can result in recovery of β cell function.