Only one of the four enantiomers of 5-fluoromethylornithine 1 was an irreversible inhibitor of ornithine aminotransferase. The active enantiomer 1a was synthesized from diaminoadipic acid 2 with a chemical diastereomeric separation and an enantiomeric resolution by hydrolysis of a phenylacetamide with benzylpenicillinase.