Infection by parasites is a major cause of production losses and mortality in young calves. The problem is most prevalent during the first grazing season (Armour, 1989). Eosinophils are important cellular mediators in immunity to gastrointestinal parasites (Baker et al, 1993), but during an extreme hypersensitivity immune response, degranulating eosinophils may lead to tissue pathology which may favour parasite survival (Miller, 1996). Dietary polyunsaturated fatty acids (PUFA) influence the fatty acid composition of immune tissues and cells (Jaffrey, 1998). n-6 and n-3 fatty acids are known to influence various components of immune response via eicosanoid dependent or independent mechanisms which influence the relative proportions of Th1 and Th2 cytokines, including IL-5 an important regulator of eosinophils maturation and recruitment. This study was carried out to establish the extent to which dietary n-6 or n-3 PUFA source affects the numbers of eosinophils in the gut of calves.