The leguminous shrub,Leucaena leucocephala(Leucaena) is wide‐spread in tropical and subtropical agricultural systems and provides a ready source of protein for livestock. However, the presence of mimosine, a non‐protein, amino acid comprising about 12% of the dry matter in growing tips ofLeucaena, is toxic to animals. Mimosine is degraded rapidly in the rumen to produce 3,4‐dihydroxypyridine (3,4‐DHP) and 2,3‐dihydroxypyridine (2,3‐DHP), both of which remain toxic to animals[1]. 3,4-DHP, as a derivative of the plant amino acid mimosine, is goitrogenic in cattle, sheep, and mice. In contrast to established antithyroid compounds, such as methimazole (MMI) and propylthiouracil (PTU), 3,4-DHP has no SH-group. 3,4-DHP with various concentrations inhibited incorporation of125I into protein in human thyroid slices. It also suppressed the activation of lymphocytes by PHA (phytohaemagglutinin) and PWM (pokeweed mitogen). Suppression with 3,4-DHP was seen at 100 and 1000 μmol/L (P< 0.001 vs both PHA and PWM). Those, together with a very low murine bone marrow toxicity, probably related to the absence of an SH-group, make 3,4-DHP a potential antithyroid drug[2].