Ninety cross-infected cases were found in bacillary disentery patients admitted in our hospital during 2 years, 1952 to 1954. Development of dysenteric symptoms is observed in a few cases when the cross-infection occurs between the bacillary types belonging to St. flexneri (B group). On the other hand, its morbidity rate is very high on the occasion of such cross-infection between different groups of Shigella genus, as from Sh. sonnei to Sh. flexneri or from Sh. flexneri to Sh. sonner (Table 1). The difference of the morbidity rate will be interpreted as follows: immunity is not formd in the case of cross-infection between different bacillary groups, but it is formed in one between same bacillary group.Whenever the secondary bacilli appear in exchange for the primary, the primary Shigella bacilli by which symptoms are caused disappear rapidly from feces, even though they are discharged continuously. The secondary bacilli already appear into the feces previous to the development of dysenteric symptoms (Fig 1).In the serum of the patients infected crossly, the agglutinin and the protective antibody against primary agent only arise in the stage of first onset, and those antibodies against the secondary one begin to arise after the second infection when the infection is caused by the bacilli of a different group from the primary one (Fig 2, 3).An attempt was made to inject Sh. flexneri 2b vaccine treated with KCN-VK3 by Hosoya's method to dysenteric patients affected with Sh. sonnei, to know whether the antibodies against Sh. flexneri arises or not and whether the cross-infection to Sh. flexneri is prevented or not. We believe the vaccination is effective from the result that the cases of the cross-infection to Sh. fiexneri is less among vaccinated patients than non-vaccinated ones (Fig 4, Table 5).