The influence of chemoprophylaxis on the immune response to gastrointestinal nematodes was evaluated in three groups of ten first-season grazing calves. Group C functioned as untreated controls, whereas Group L received two treatments, one at turn-out and one 8 weeks later, with doramectin (200 micrograms kg-1) and Group H three treatments, 3, 8, and 13 weeks post turn-out, with ivermectin (200 micrograms kg-1). At housing, five calves of each group were randomly selected for necropsied for worm counts. The remaining calves were challenged with 50,000 Ostertagi and 100,000 Cooperia oncophora and necropsied 4 weeks later. During the grazing season, Group C suffered from an outbreak of parasitic gastroenteritis which necessitated two anthelmintic treatments. In both treated groups no clinical symptoms were observed and growth rates were normal. Pepsinogen levels and antibody concentrations indicated a moderate and low gastrointestinal nematode infection in Group L and Group H, respectively. The Ostertagia and Cooperia worm populations present in the calves necropsied at housing were reduced in both treated groups in comparison with Group C. In the challenged calves, egg output was significantly higher in Group H than in Group L and Group C. Ostertagia and Cooperia worm burdens after challenge were high in Group H, moderate to low in Group L and low in Group C. This study indicates that highly suppressive prophylactic programmes, such as the ivermectin 3, 8 and 13 weeks schedule, can impair immune development against gastrointestinal nematodes during the first grazing season.