Insulin B chain peptide B:9-23 given with incomplete Freund's adjuvant (IFA) subcutaneously to NOD and BALB/c mice induces insulin autoantibodies (IAA). We also found that subcutaneous administration of the peptide without adjuvant induced IAA in normal BALB/c mice. The autoantibodies react with intact insulin and cannot be absorbed by the B:9-23 peptide. With the induction of IAA by the self-peptide without adjuvant, we hypothesized that the peptide given subcutaneously without adjuvant would prevent the development of diabetes mellitus in NOD mice. The peptide B:9-23, when given in standard doses of 100 microg and low doses of 10 microg, protected female NOD mice versus unvaccinated controls from diabetes. Presently, NOD mice vaccinated with the standard dose and the low dose have a 44% and 60% survival, respectively, at 26 weeks compared to controls with a 10% diabetes-free survival at 22 weeks (n = 10 for each group, P < 0.001 for both vaccine doses). As expected, the level of IAA expressed was significantly higher for the vaccinated mice versus the control group. We conclude that insulin B chain peptide B:9-23 can confer protection from diabetes in NOD mice even when administered subcutaneously without adjuvant.