The properties of a specific system for the transport of S -adenosylmethionine in yeast are described. The process was p H-, temperature-, and energy-dependent, and showed saturation kinetics. The Km for the system was 3.3 × 10 −6 m . Of the S -adenosylmethionine moieties tested, only S -adenosylhomocysteine competitively inhibited the uptake of the adenosylsulfonium compound. Adenine, adenosine, methionine, homocysteine, and the sulfonium compound S -methylmethionine were without effect. The analogue S -adenosylethionine showed competitive inhibition. Under conditions of inhibition of protein synthesis by cycloheximide or methionine starvation, permease activity was stable. The mutant sam-p3 apparently was able to transport S -adenosylmethionine only by diffusion. Uptake by diploids containing this mutation was directly proportional to the gene dose.