FROM recent work1 it would seem that L-serine ethanol-amine phosphodiester (L-SEP) is found only in the tissues of fish, amphibians, reptiles and birds. D-SEP, on the other hand, appears to be confined to earthworms2, where it is the biological precursor of lombricine3. The status of the work on both isomers has been summarized by Ennor and Rosenberg4. These authors had previously shown1 that dinitrophenol (3 × 10−4 M) completely inhibited the biosynthesis of L-SEP and that cytidine triphosphate (CTP) appeared to be required. Thus it was shown that chicken kidney homogenates, supplemented by CTP, incorporated more inorganic phosphorus-32 into L-SEP than when this nucleotide was omitted.