The effectiveness of many vaccines licensed for clinical use relates to the induction of neutralising antibodies, facilitated by the inclusion of vaccine adjuvants, particularly alum. However, the ability of alum to preferentially promote humoral rather than cellular, particularly Th1-type responses, is not well understood. We demonstrate that alum activates immunosuppressive mechanisms following vaccination, which limit its capacity to induce Th1 responses. One of the key cytokines limiting excessive immune responses is IL-10. Injection of alum primed draining lymph node cells for enhanced IL-10 secretion ex-vivo. Moreover, at the site of injection, macrophages and dendritic cells were key sources of IL-10 expression. Alum strongly enhanced the transcription and secretion of IL-10 by macrophages and dendritic cells. The absence of IL-10 signalling did not compromise alum-induced cell infiltration into the site of injection, but resulted in enhanced antigen-specific Th1 responses after vaccination. In contrast to its decisive regulatory role in regulating Th1 responses, there was no significant change in antigen-specific IgG1 antibody production following vaccination with alum in IL-10-deficient mice. Overall, these findings indicate that injection of alum promotes IL-10, which can block Th1 responses and may explain the poor efficacy of alum as an adjuvant for inducing protective Th1 immunity. This article is protected by copyright. All rights reserved.