Reduction with sodium in liquid ammonia ofl-thiazolidine-4-carboxylic acid (II) leads, depending on the reaction procedure followed, toN-methyl-l-cysteine (I) or a mixture ofN-methyl-l-cysteine (I) (30%) and a dimer (50%), the structure of which is shown to be VI. Contrary to what has been expressed in the literature, the reaction of 1,2-dichloroethane with cysteine (III) at a pH of 8–10 does not lead to compound VI but to compound VIII.