Five Shigella strains isolated from stool cultures of two domestic dysentery cases and three imported cases in Tokyo in 1986-1989 had an atypial serotype that agglutinated with only antiserum to S. flexneri group 6 factor antigen. These strains showed typical biochemical reactions of S. flexneri and were biochemically identical. Four of the strains were positive for Serény test with guinea pig eye and the cell-invasion test with HeLa cells. The strains also had the virulence-plasmid encoding outer membrane proteins, indicating that these were pathogenic. These strains were serologically identical and gave significant cross-reactions with S. flexneri 1b and 4b that had group 6 factor antigen. However, results of reciprocal absorption tests showed that the O antigen of these strains were not identical to S. flexneri 1b and 4b, and were confirmed to have a new type antigen which can not be included among the known S. flexneri type antigen I-VI. Strain 88-893 is designated as the test strain for this new type antigen of S. flexneri.