Comparison of [U-14C]serine and [3-14C]serine as precursors of the alanine side-chain of the pyrimidinyl amino acid lathyrine, indicate that serine donates all three of its carbon atoms as a unit. Isotope incorporation studies identified the precursor of the pyrimidine moiety as 2-amino-4-carboxypyrimidine. Studies with enzymic extracts of Lathyrus seedlings showed the presence of a pyridoxal phosphate-dependent lathyrine synthase which simultaneously decarboxylates the pyrimidine precursor and alanylates its C-4 position. The enzyme is stimulated by biotin. Unlike a number of other plant synthases catalysing formation of heterocyclic β-substituted alanines, lathyrine synthase requires serine and not O-acetylserine as a substrate. The action of the synthase was successfully modelled in a nonenzymic pyridoxal-catalysed reaction in the presence of either Al3 + or Ga3 + ions. The latter were much more effective in this respect.