One of the most important aspects in vaccine development is to induce potent antigen-specific immune responses. In this study, we examined the immunological activities of antigen-carrying biodegradable poly(gamma-glutamic acid) (gamma-PGA) nanoparticles (NPs) in mice. The immunization with ovalbumin (OVA)-carrying gamma-PGA NPs (OVA-NPs) could induce significant expansion of antigen-specific CD8(+) T cells. Unlike complete Freund's adjuvant, subcutaneous (s.c.) inoculation of OVA-NPs to footpad did not generate injection site swelling. Although OVA-NPs could induce both antigen-specific cellular and humoral immune responses, the dominant induction of either cellular or humoral immunity was found to depend on their administration routes. Strong antibody production was observed by s.c. immunization, yet no antibody was identified by intranasal immunization. Thus, gamma-PGA NPs are a safe and efficient antigen carrier with unique immunological properties. (C) 2008 Elsevier Inc. All rights reserved.