cit.), which reflects their binding to lipids in the worms. Despite the activity of glutathione transferase, the complete recovery of the unchanged TBZ from (14C) TBZ in in vitro uptake studies indicates that this enzyme plays no role in detoxification of TBZ. The worms, however, were able to conjugate 5-OH TBZ to a limited extent; the capacity for sulfation of xenobiotics has not been reported previously. There was no evidence that either drug uptake or drug metabolism was altered in the TBZ-resistant strain of T. colubriformis used for the present study. TBZ was present in intestinal mucus at levels considerably higher than those in plasma. These mucus levels may have been augmented by drugs from bile and intestinal secretions. The final concentration within the worms was higher again, presumably due to a passive absorption of drug into worm lipids, and corresponded to approximately 20 uM after a 50 mg/kg treatment of infected sheep. This work was supported by the Australian Wool Research Trust Fund. A portion of this work was conducted at the CSIRO, McMaster Animal Health Laboratory, Glebe, N.S.W., Australia.