The consequence of sub-clinical gastrointestinal parasitism in farm animals, i.e. reduced food intake and growth, is often more pronounced in breeds with a high production potential compared with breeds with a low one (Houdijk et al., 2001). It cannot be excluded that such differences in disease resistance may arise from between-breed differences in genetic resistance to parasites rather than production potential per se, as within-breed selected farm animals with sufficiently different production potentials are not readily available. However, appropriately selected mouse lines are available (Bünger et al., 2001) to explore the hypothesis that selection for growth may reduce the animal’s capacity to cope with pathogens. We have recently shown in mice divergently selected for growth potential that in some lines, the absolute penalty of sub-clinical parasitism on high growth mice was higher than in their low growth counterparts (Houdijk and Bünger, 2006). Here, we test the hypothesis that this penalty on growth in one of these lines can be reduced through increased protein supply.