Several methods for the chemical and enzymatic synthesis of (-)-S-adenosylmethionine (AdoMet) are described and compared. Studies on the effects of solvents, pH, methylating reagents, and KI on the coupling of sodium homocysteine thiolate and 5'-chloro-5'-deoxyadenosine led to an improved procedure for the synthesis of (+/-)-AdoMet. The use of trimethylsulfonium iodide as a methylating agent under acidic conditions results in a higher content of the desired (-)-epimer than does the use of CH3I. The enzymatic synthesis of (-)-AdoMet using AdoMet synthetase from an over-producing strain of Escherichia coli is demonstrated and the effect of product inhibition on preparative-scale synthesis is illustrated. A new HPLC technique for separation of the epimeric mixture of AdoMet, which allows small-scale preparation of optically pure AdoMet from the enzyme product, has been developed. With this HPLC technique, evidence that (-)-AdoMet is the sole epimer formed by the enzyme has been shown.