2,3-Diaminobutyric acid (DABA) is an unusual di-amino acid component of mureidomycin A, a member of the peptidylnucleoside antibiotic family produced by Streptomyces flavidovirens SANK 60486. Radiochemical assays using cell-free extract from S. flavidovirens revealed that 14C-L-Thr is converted into radiolabelled DABA by an ammonia-dependent beta-replacement activity, and not via oxidation to 3-keto-2-aminobutyric acid. The substrate specificity of partially purified enzyme was assayed using a spectrophotometric assay, and beta-replacement activity was inhibited by known inhibitors of PLP-dependent enzymes. These data imply that DABA is biosynthesised from L-Thr by a PLP-dependent beta-replacement enzyme, using ammonia as a nucleophile. These results are consistent with literature proposals for the biosynthesis of 2,3-diaminopropanoic acid from the viomycin biosynthetic gene cluster.