Little is known about the immunological impact of insulin administration other than it can boost insulin autoantibody levels. In particular, while the subcutaneous administration of a soluble foreign antigen (without adjuvant) is generally only weakly immunogenic in a naive animal, it is unknown what effect the subcutaneous administration of a soluble self-antigen has in animals with established autoimmune responses to the antigen. Addressing these questions in pre-diabetic nonobese diabetic (NOD) mice, we examined the effects of administering insulin, as well as the metabolically inactive B-chain of insulin, on insulin-specific cellular and humoral immune responses. We show that pre-diabetic NOD mice have a spontaneous Th1-biased response against insulin. Administering insulin, or the insulin B-chain, rather than boosting the established Th1 response, primed Th2 cellular and humoral immunity to insulin, shifting the predominant insulin response toward a Th2 phenotype. Despite the presence of a Th1 response against insulin, insulin treated mice failed to mount proliferative T-cell responses following immunization and challenge with insulin, demonstrating that the treatment induced an active form of tolerance to this autoantigen. Thus, the subcutaneous administration of a soluble antigen can engage Th2 responses and induce self-tolerance, even after the establishment of autoreactive Th-1 responses. Such immune deviation and induced regulatory tolerance may contribute to the protective effects of prophylactic insulin therapy, as well as the establishment of a "honeymoon" phase in new-onset insulin-dependent diabetic patients.