Background: The development of nanoparticulate antigen-delivery systems is an important emerging area of vaccinology, being sought to amplify immune responses to recombinant antigens that are poorly immunogenic. Nanoparticle size may play an important role in influencing the activity of such particulate-based adjuvants. Methods: To explore how the size of nanoparticles that are in the range of many common viruses can modulate the magnitude and quality of mucosal immune responses, the model antigen ovalbumin (OVA) was conjugated to 30 nm or 200 nm polypropylene sulfide nanoparticles (NPs) and administered intranasally to C57BL/6 mice. Results: We show that by increasing the size of the NPs from 30 to 200 nm, OVA was more effectively delivered into both MHC class I and MHC class II-presentation pathways. Intranasal immunization with the 200 nm NPs increased the magnitude of CD4(+) T cell responses in the lungs, as well as systemic and mucosal humoral responses. Most importantly, 200 nm NPs increased the proportion of antigen-specific polyfunctional CD4(+) T cells as compared to 30 nm NPs. Conclusions: The 200 nm NPs are a very interesting antigen nanocarrier for prophylactic vaccines against mucosal pathogens that require multifunctional CD4(+) T cells for protection. These results contribute to our understanding of how the size of an antigen-conjugated nanoparticle modulates mucosal immune responses to a protein antigen and may be useful to engineer subunit vaccines able to elicit appropriate mucosal immune responses that correlate with protection. (C) 2012 Elsevier Ltd. All rights reserved.