The imminent ban on the use of antibiotics and antibiotic growth promoters in animal feed in EU member states from 1st January 2006 provides a new focus for the development of alternative strategies to augment immune function in farm animals. In recent years there has been an increased interest in the ability of certain nutraceuticals to enhance immune response and consequently improve health status. For example, there is now some evidence for non-ruminants that consumption of polyunsaturated fatty acids (PUFA) including eicosapentaenoic acid (EPA; 20:5n-3), docosapentaenoic acid (DPA; 22:5 n-3) and docosahexaenoic acid (DHA; 22:6n-3), belonging to the n-3 fatty acid series, can affect immune function. However, both positive and negative effects have been reported depending on the specific fatty acid and amount used (see review by Calder, 1998). There is a dearth of information, however, on the possible effects of PUFA on immune status in ruminants. One recent study (Wistuba et al., 2005) suggests that fishoil may act as an immune stimulant in grazing beef calves. The objective of the current study was to examine the effect of level of dietary omega-3 fatty acid supplementation on the haematological profile and indicators of immune function in cattle.