Live Shigella flexneri 2a vaccine candidate strains with deletion mutations in the aro genes were constructed. Tn10-generated auxotrophic mutations were transduced from Escherichia coli to S. flexneri 2a with bacteriophage P1CmCts. The tetracycline-sensitive derivatives of Tn10 mutants obtained were selected on Bochner's medium and checked by DNA-DNA hybridization using aroA and aroD gene specific probes. The vaccine candidate strains were tested to assess the efficacy of protection in guinea-pig conjunctival epithelia (Sereny test). The strains did not cause keratoconjunctivitis and exhibited significant protection in the challenge experiments. A candidate vaccine strain (delta aroD) showed 100% protection against 10(7) c.f.u. of wild type strain in the immunized guinea-pigs.