Possible mechanisms in the biosynthesis of lathyrine are discussed. An in vitro system for the specific C hydroxylation of diamino acids is reported and has been compared with the conversion of homoarginine to 4-hydroxyhomoarginine by Luthyrus tingitanus. The synthesis of trans4,5dehydro-o,L-homoarginine, a new amino acid is described. [guanidino-'4C]trans-4,5-Dehydro-n,L-homoarginme was not converted to [14C]lathyrine when supplied to L. tingitumu. Lathyrine has been synthesised from Coxolysine via Coxohomoarginine. It is suggested that Coxohomoarginine is the direct precursor of lathyrine.