We analyzed the phenotype and function of bone marrow-derived dendritic cells (DCs) induced in vitro without using any serum during the late stage of cultivation. These 'serum-free' DCs (SF-DCs) possessed the ability to induce T cell proliferation as well as antibody responses, indicating that they were functional DCs. Surprisingly, the SF-DCs akin to semi-mature DCs in terms of both phenotypic and functional characteristics. The SF-DCs did not produce IL-12 but produced large amounts of IL-23 following lipopolysaccharide Stimulation. The antigen-specific production of IL-17 by CD4(+) T cells co-cultured with OVA-loaded SF-DCs was significantly higher than that with OVA-loaded conventional DCs. These results suggest that SF-DCs tend to produce IL-23 and can consequently induce the IL-17 producing CD4(+) T cells. The semi-mature DC-like cells reported here will be useful vehicles for DC immunization and might contribute to studies on the possible involvement of semi-mature DCs in Th17 cell differentiation. (C) 2009 Elsevier Inc. All rights reserved.