Castellani isolated from the feces of patients with clinical dysentery a bacterium which he named “Bacillus ceylonensis B.” Five years later he reported the isolation from normal human stools of Bacillus madampensis. Both organisms were gram-negative, nonmotile rods, fermenting many carbohydrates, including lactose, with production of acid only, and producing indol. Bacillus ceylonensis B fermented dulcitol, whereas Bacillus madampensis did not. Both fermented sucrose. Andrewes included these 2 species, together with some other lactose-fermenting dysentery-like varieties, in his “Bacillus dispar.” Current terminology places these microorganisms in the genus Shigella. Castellani stated that Bacillus ceylonensis B was a serologically homogeneous species. Glynn and Starkey, working with 15 strains, reported Shigella dispar antigenically heterogeneous. The present investigation was undertaken to study further the antigenic structure of strains designated Shigella dispar or Shigella ceylonensis. Thirty-seven strains of bacteria are included in this preliminary report. Five were received from the American Type Culture Collection; 10 were received from Dr. K. M. Wheeler, and were isolated in Connecticut within the past 4 years; 3 were isolated in Texas in 1942, and were received from Dr. MacDonald Fulton; two were isolated recently in Rhode Island by Dr. C. A. Stuart; the remaining 17 were isolated in a number of different laboratories along the Atlantic seaboard. Eleven strains of Sh. ceylonensis were received, and 26 strains of Sh. dispar. All 37 strains produced acid from dextrose, lactose, maltose, and mannitol; failed to ferment salicin, and were indol positive. Further fermentation reactions are noted in Table I. Antiserums were prepared for 2 strains of Sh. dispar and one of Sh. ceylonensis. Table I presents a summary of the results obtained when the 37 strains were tested for agglutination in the various antiserums. The titers stated are those for the type strains listed, but all organisms grouped together behaved similarly.