Based on our previous findings we postulate that the production of blood group B-degrading mucinase by Shigella flexneri 2a is related to virulence. The virulent S. flexneri 2a strain M4243 produced a blood group B mucinase which decreased the blood group B reactivity of germ-free mouse mucins by a factor of 16 and the B reactivity of human saliva by a factor of 32. Avirulent S. flexneri 2a B-1, serologically similar, but not genetically identical to the M4243 strain, failed to degrade the blood group reactivity. The mucin-degrading ability of S. flexneri 2a M4243 harboring a large virulence-conferring 140 MDa plasmid was then compared with a genetically similar large plasmid-free avirulent S. flexneri 2a M4243A1. Virulent S. flexneri M4243 grew in human salivary mucins while the genetically identical avirulent M4243A1 did not. Supernatant of virulent M4243 culture decreased the blood B reactivity of salivary mucins by a factor of 32 while the avirulent M4243a1 had no effect. Eleven of 12 colonies of the transconjugant hybrid Escherichia coli K12 (7300-1-5) containing the shigella PWR 110 plasmid and chromosomal markers decreased the blood group B reactivity by a factor of 4-32, and two of four colonies of the E. coli strain 7262 containing only the plasmid reduced the B reactivity by a factor of 4-16. These findings suggest that blood group B-specific mucinase production may be related to S. flexneri virulence.