Abstract [3 H] RU38486 (RU486) was synthesized from its penultimate precursor, desmethyl RU486, by substitution of the secondary amine with tritiated methyl iodide; specific activity 85 Ci/mmol. Ligand binding studies confirm that RU486 binds with high affinity to human progesterone receptor type A (PR-A) and progesterone receptor type B (PR-B).