Heat shock protein 90 (HSP90) is a molecular chaperone required for efficient antigen presentation and cross-presentation. In addition, HSP90 was recently reported to interact with and stabilize the activation-induced cytidine deaminase (AID) and plays a critical role in immunoglobulin gene hypermutation and class switch recombination. In mice and humans, there are two HSP90 isoforms, HSP90 and HSP90, but the in vivo role of each isoform remains largely unknown. Here we have analyzed humoral immune responses in HSP90-deficient mice. We found that HSP90 deficiency did not affect AID protein expression. B cell development and maturation, as well as immunoglobulin gene hypermuation and class switch, occurred normally in HSP90-deficient mice. However, antibody production to a T-dependent antigen was elevated in the mutant mice and this was associated with enhanced MHC class II antigen presentation to T helper cells by dendritic cells. Our results reveal a previously unidentified inhibitory role for HSP90 isoform in MHC class II antigen presentation and the humoral immune response. Along with our recent finding that HSP90 is required for antigen cross-presentation, these results suggest that HSP90 controls the balance of humoral and cellular immunity by dictating the fate of presentation of exogenous antigen.