N -(mercaptomethyl) -phthalimide-carbonyl-14C-S-(0,O- Dimethylphosphorodithioate) has been synthesized. The starting material was carboxyl labelled o-toluic acid which was oxidized to o-phthalic acid. Phthalimide (carbonyl-W) was prepared from o-pkthalic acid by subliming the ammonium salt of the acid. When 14C phthalimide was reacted with aqueous formaldehyde, N-hydroxymethyl phthalimide was formed. The final product was obtained when the N-hydroxymethyl derivative was reacted with thionyl chloride, followed by reaction with sodium dimethyl phosphorodithioate.