The Reg family of proteins has been studied in the context of growth and regeneration in several organs including pancreatic islets. We previously suggested that Reg proteins act as autoantigens in type 1 diabetes, based on evidence that a member of the Reg family (hepatocellular carcinoma intestine pancreas [HIP]/pancreatitis-associated protein [PAP]) was overexpressed in the islets of a patient who died after sudden onset of type 1 diabetes, and that, in NOD mice, Reg-specific T-cells adoptively transferred diabetes. In the current study, we developed antisera to detect individual Reg members in mouse islets and found that RegIIIalpha was present in the non-beta-cell portion of the islets, while RegII was predominantly expressed in beta-cells. Vaccination of NOD mice with the separately expressed N-terminal (NtfrII) or C-terminal (CtfrII) portion of RegII revealed a dichotomy: NtfrII vaccination accelerated and CtfrII vaccination delayed type 1 diabetes. Vaccination with CtfrII was more effective when given at later stages in the pathogenesis of type 1 diabetes, a time dependency different from that seen with other antigen-dependent vaccine strategies in NOD mice, which might have therapeutic implications. In conclusion, RegII is a novel beta-cell-derived autoantigen in NOD mice. The autoimmune response against this protein may convert a regenerative into an islet-destructive process accelerating development of type 1 diabetes.