The present study was undertaken to investigate the effects of extracts of diesel exhaust particles (DEP) on Th1 and Th2 immune responses. In order to separate compounds from DEP different in hydrophobicity, a single DEP sample was consecutively extracted with hexane (HEX-DEP), benzene (BEN-DEP), dichloromethane (DIC-DEP), methanol (MET-DEP), and 1M ammonia (AMM-DEP). The last unextracted residue (UNE-DEP) was also used to test its effect on immune responses. To immunize mice, hen egg lysozyme (HEL) was injected i.p. (day 0). Varying doses of DEP, each DEP extract, and UNE-DEP were intranasally administered every 2 days from days 0 to 18. Anti-HEL IgG2a antibodies in sera and IFN-γ secreted from spleen cells were measured as an indicator of Th1 immune responses, while anti-HEL IgG1 antibodies and IL-4 as that of Th2 responses. The results showed that treatment with DEP and DIC-DEP increased both Th1 and Th2 responses to HEL. UNE-DEP facilitated Th1 but not Th2 responses, while MET- and AMM-DEP administration was followed by enhancement of Th2 but not Th1 responses. Neither HEX- nor BEN-DEP modulated Th1 as well as Th2 responses. These results suggest that DEP contain various compounds different in hydrophobicity which may affect both Th1 and Th2, Th1 but not Th2, and Th2 but not Th1 immune responses.