Eosinophil and IgE responses of interleukin (IL)-5 transgenic and normal C3H/HeN mice were studied after experimental infection with Nippostrongylus brasiliensis (Nb). Intestinal worms were recovered at day 5 post-infection (PI), and numbers of total white blood cells (WBC) and eosinophils, and total serum IgE and anti-hapten (dinitrophenyl) (DNP) specific IgE titers, were measured at days 0, 14 and 21 PI. IL-5 mice appeared resistant to Nb infection showing a significantly lower worm recovery rate than normal mice (P<0.05). Total WBC and eosinophil counts (/mm³) were significantly increased in Nb infected normal mice (P<0.05), but unchanged (total WBC) or decreased (eosinophils) in IL-5 mice at day 21 PI. The total serum IgE level remarkably increased in normal mice, but only a little in IL-5 mice at days 14 and 21 PI. Priming with DNP brought about more remarkable increases of the total and anti-DNP specific IgE in normal mice than in IL-5 mice. The results show that IL-5 mice are resistant to Nb infection, and that eosinophil and IgE responses in these mice are not augmented by Nb infection.