Bovine milk contains a variety of proteins and peptides that are biologically active (Ogra & Ogra, 1978; Duncan & McArthur, 1981; Newby et al. 1982; Juto, 1985; Stoeck et al. 1989; Mincheva-Nilsson et al. 1990; Watson, 1990; Barta et al. 1991; Politis et al. 1991; Fiat et al. 1993). Our laboratory has a long-term interest in some purified milk proteins, particularly lactoferrin (LF), lactoperoxidase (LP) and β-casein (β-CN), which have been shown to be immunologically significant. Some of our recent studies on these bovine milk proteins, particularly β-CN, indicated that their in vitro immunological effects did not always parallel their in vivo activities (Wong et al. 1996a, b; 1997a, b). This study was designed to investigate and compare the capacity of these purified bovine milk proteins to modulate a range of components that are vital to in vivo immune responses in sheep, with a view to providing further information on their potential in biomedical applications. To achieve this objective, a sensitive lymphatic cannulation model was employed that allows in vivo immune components and their functions to be measured in lymph collected under physiological conditions.