A field study was carried out on Little Island (County Waterford, Ireland) in June 2000 to evaluate the potential of a bait-marking system for use in European badgers (Meles meles). Two oral biomarkers, sulfadimethoxine (SDM) and rhodamine B, were incorporated into fishmeal baits and distributed by hand at main sets in five test territories for 3 consecutive days. In parallel, non-biomarked baits were distributed at a single control territory. The objectives of the study were to: (1) assess the effects of SDM and rhodamine B on palatability and thus bait acceptance, and (2) investigate the marking capacity of SDM and rhodamine B in serum and hair samples taken from badgers. Trapping was carried out in each territory for 5 consecutive days immediately after bait distribution. Analysis of data revealed that 90-100% of baits were removed in four of the test territories and from the control territory. In the fifth test territory, 61% of baits were removed. Of the badgers (n = 26) trapped in the test territories, 18 (69%) were positive when tested for both biomarkers. In contrast, the remaining eight animals and those captured in the control territory (n = 6 badgers) were negative. In the marked animals, the highest levels of SDM were recorded in serum samples taken soon after bait distribution. Thereafter, the levels declined in each badger over the course of the study. In contrast, rhodamine B was readily detectable by fluorescence microscopy of hair samples throughout the period of study. The results indicate that SDM and rhodamine B act as systemic markers in badgers and have potential future applications for monitoring of oral vaccine uptake.