It has been shown that pulmonary exposure to diesel exhaust particles (DEP) disrupt immune systems, presenting as exacerbating effects on allergic manifestations (e.g., allergic asthma). To date, the impact of nano-level DEP on health has not been fully elucidated. Our institute (the National Institute for Environmental Studies) established an 'environmental nanoparticle exposure system applied in animals' in 2005 and, since then, the health effects of exposures to these types of agents have been explored. The present study was designed to investigate the in vitro effects of nanoparticle-rich DEP (NRDEP) on primary splenocytes from atopy-prone hosts. NC/Nga mouse-derived splenic mononuclear cells were co-cultured with NRDEP (0-50 µg/ml); thereafter, cell viability/proliferation was evaluated via a WST-1 assay, production/release of interleukin (IL)-17A in the culture supernatants by ELISA, and expression of RORγt (retinoic acid-related orphan receptor-γt) in cell lysates by Western blot analyses. The results indicated that NRDEP reduced cell viability/proliferation in a dose-related manner-significantly so at a level of 50 µg/ml NRDEP. In contrast, up to 10 µg NRDEP/ml increased RORγt expression in the splenocytes and subsequent IL-17A production/release by the cells in a dose-dependent manner with an overall trend (with significance vs 1 µg NRDEP/ml and 10 µg NRDEP/ml for IL-17A); 50 µg NRDEP/ml tended to inhibit the transcription factor expression and cytokine production/release. These results suggest that NRDEP can activate naïve splenic mononuclear cells from atopy-prone animals in terms of RORγt and IL-17A induction (T(H)17 response).