13-cis-Retinoic acid is a stereospecific suicide inhibitor of thioredoxin reductase. [3H]-labeled 13-cis-retinoic acid has been used to covalently label thioredoxin reductase in human keratinocytes. The acid-soluble cytosol fraction of human keratinocytes contained three radioactive proteins labeled by the addition of high-specific-activity [3H]-13-cis-retinoic acid to cell cultures. One of these proteins was identified as cytosolic keratinocyte thioredoxin reductase by fast-protein liquid chromatography and SDS-gel radioautography. The inhibition of thioredoxin reductase by 13-cis-retinoic acid may explain the known cytostatic and teratogenic properties attributed to this retinoid.