Trapoxin, a cyclotetrapeptide isolated from the fungus Helicoma ambiens, profoundly affects mammalian cell growth and morphology. Ln this paper, we describe syntheses of trapoxin, [H-3]trapoxin, and K-trap, a trapoxin-based affinity reagent. These reagents have allowed us to achieve the first molecular characterization of histone deacetylase, trapoxin’s cellular target.