A Tn5-generated mutant (strain S2430) of Shigella flexneri 2a (strain YSH6000) exhibited attenuated virulence and, in addition to the Tn5 insertion in the SalI K fragment of its virulence plasmid, had a 99-kb deletion within its chromosome. Unlike its wild-type parent, strain S2430 was susceptible to ampicillin, streptomycin, tetracycline and chloramphenicol. An independent multi-antibiotic susceptible variant of strain YSH6000 had a similar deletion. Southern blot analysis of pulsed field electrophoresis gels enabled the sizing of this deletion and its mapping to a region of the chromosome on NotI fragment D bounded by the S. flexneri homologues of ompA and pyrC. Hybridisation experiments with a probe specific to the multi-antibiotic resistance region indicated that this large deletion was responsible for antibiotic susceptibility. Both strain S2430 and a derivative of the antibiotic-susceptible variant, with a Tn5 insertion in its SalI K fragment, exhibited an equal reduction in contact haemolysis compared with the Tn5-bearing derivative of strain YSH6000. However, strain S2430 alone clearly displayed delayed plaque forming ability in LLC-MK2 monolayers, suggesting that the two examples of this deletion may not be identical.