The anthelmintics bithionol, phenothiazine, albendazole and fenbendazole were oxidized to sulphoxides by enzymes in the cytosol of the proglottids of the cestode Moniezia expansa and the cytosol of the intestinal epithelial cells of the nematode Ascaris suum. Enzymes in these tissues were also able to reduce these sulphoxides to the thioethers in the absence of oxygen. 2. Sulphoxidation and sulphoxide reduction also occurred in mouse liver enzyme preparations. About 20% of the sulphoxidation activity was not associated with microsomes and was not inhibited by CO; about 50% of the reductase activity was found in the microsomes. 3. The pH optima for sulphoxidases from both helminths were in the range 7.0--7.2, and both required NADH or NADPH for activity. Low molecular weight thiols and flavins did not affect sulphoxidation. Enzyme activity was inhibited by 0.1 mM Cu2+, Hg2+, Cd2+ or Zn2+ and by p-chloromercuribenzoate or N-ethylmaleimide. 4. Both helminth sulphoxide reductases displayed pH optima in the range 1.2--7.4, and required NADH or NADPH for activity. Oxygen inhibited the reductases.
