Certain thiol-containing compounds catalyze, in a chemical reaction, the isomerization of 9-cis-retinoic acid to a mixture of all-trans-retinoic acid, 9-cis-retinoic acid, 13-cis-retinoic acid, and 9,13-dicis-retinoic acid. In the presence of such catalysts, all-trans-retinoic acid gives rise to the same mixture. Reactions approaching equilibrium contain more all-trans-retinoic acid than either of the other isomers. Small molecules effective as catalysts are mercaptoethanol, L-cysteine methyl ester, glutathione, and N-acetyl-L-cysteine. Apoferritin (a thiol-containing protein), native microsomes, and, to a lesser extent, boiled microsomes catalyze the reaction. In intact cells, these interconversions also occur in a process inhibited by a sulfhydryl-specific reagent. The thiol-catalyzed isomerization of 9-cis-retinoic acid may be relevant in the biological activity of this compound.