Helminthologically naive 8-week-old lambs were administered 50000 infective-stage larvae of the trichostrongyle nematode Nematodirus battus. Initial mean adult worm burdens were significantly reduced (P < 0·01) at day 21 post-infection (p.i.). Peripheral blood eosinophil counts showed a significant increase (P < 0·001) by day 25 p.i. when compared with uninfected control animals. Sections of small intestine taken at post-mortem from the area of adult worm establishment over the course of infection showed a significant increase in mucosal eosinophil and mast cell numbers during the period of adult worm rejection, when compared with uninfected control animals. These results show that young lambs develop significant increases in the numbers of cells associated with an inflammatory reaction in the intestine during the development of a partially protective immune response to this parasitic nematode.