Particulate adjuvants have shown increasing promise as effective, safe, and durable agents for the stimulation of immunity, or alternatively, the suppression of autoimmunity. Here we examined the potential of the adjuvant carbonyl iron (CI) for the modulation of organ‐specific autoimmune disease—type 1 diabetes (T1D). T1D was induced by multiple low doses of streptozotocin (MLDS) that initiates beta cell death and triggers immune cell infiltration into the pancreatic islets. The results of this study indicate that the single in vivo application of CI to MLDS‐treated DA rats, CBA/H mice, or C57BL/6 mice successfully counteracted the development of insulitis and hyperglycemia. The protective action was obtained either when CI was applied 7 days before, simultaneously with the first dose of streptozotocin, or 1 day after MLDS treatment. Ex vivo cell analysis of C57BL/6 mice showed that CI treatment reduced the proportion of proinflammatory F4/80⁺CD40⁺ M1 macrophages and activated T lymphocytes in the spleen. Moreover, the treatment down‐regulated the number of inflammatory CD4⁺IFN‐γ⁺ cells in pancreatic lymph nodes, Peyer's patches, and pancreas‐infiltrating mononuclear cells, while simultaneously potentiating proportion of CD4⁺IL17⁺ cells. The regulatory arm of the immune system represented by CD3⁺NK1.1⁺ (NKT) and CD4⁺CD25⁺FoxP3⁺ regulatory T cells was potentiated after CI treatment. In vitro analysis showed that CI down‐regulated CD40 and CD80 expression on dendritic cells thus probably interfering with their antigen‐presenting ability. In conclusion, particulate adjuvant CI seems to suppress the activation of the innate immune response, which further affects the adaptive immune response directed toward pancreatic beta cells.