Since plant vacuoles are the cellular repository for detoxification of heavy metal conjugates, herbicides and toxins (Rea et al, 1998), mastication and rumen digestion of plant based forage in bovines will release accumulated toxins within the rumen which is also the major site for absorption of short-chain fatty acids from microbial fermentation (Graham and Simmons 2005). Accordingly we have investigated the ruminal expression of bovine ABC transporters associated with cytoprotection of gastro-intestinal enterocytes against xenobiotic exposure, namely MDR1 (ABCB1), MRP2 (ABCC2) and BCRP (ABCG2) (Haslam et al, 2008). Bovine rumen samples from the ventral sac were obtained post-mortem from a commercial slaughterhouse after humane killing. Epithelial tissue was then stripped from the underlying muscle layers and samples prepared for immunohistochemistry, SDS-PAGE/ Western blotting and total RNA isolation for RT-PCR as previously described (Graham and Simmons 2005). Bovine-specific primers used for PCR were designed for MDR1, BCRP and MRP. β-actin was used as a control transcript. Using 30 cycles PCR products of the predicted size were observed for both MDR1 and BCRP, but not for MRP2. Figure 1A shows a Western blot of MDR1 expression using C219 primary monoclonal antibody (mab) in bovine rumen with a doublet at an apparent molecular size of ~ 170-180 kD. Immunolocalisation of MDR1 using UIC2 mab within cryosections of bovine rumen confirms MDR1 expression. The tissue was co-stained with the nucleic acid marker ethidium homodimer 1 with the overlay combining MDR1/nucleic acid staining. MDR1 expression is absent from outer stratum corneum but extensive membrane staining is observed in cells of the stratum granulosum, stratum spinosum and stratum basale. Protein expression and immunolocalisation was also confirmed for BCRP, with prevalent staining in the stratum basale, becoming weaker in the stratum spinosum and stratum granulosum (not shown). No MRP2 expression was apparent as confirmed by RT-PCR, immunostaining and western blot. We conclude that extensive expression of both MDR1 and BCRP within bovine rumen may play a role in cytoprotection of ruminal epithelium against xenobiotics present in forage.