Synthetic decarboxylated S-adenosylmethionine and six analogs were tested in a decarboxylating system based on rat liver S-adenosylmethionine decarboxylase. All the compounds inhibited the putrescine-activated decarboxylase activity and were competitive with respect to S-adenosylmethionine. Among the compounds tested, S-5'-deoxyadenos-yl-(5')-2-methylthioethylamine was the most potent inhibitor. The K_i value of the inhibitor was seven times lower than that of decarboxylated S-adenosylmethionine.