It is a well known fact that Vi antigen possesses suppressive power on O hemagglutinability. So far, not a few experiments have been done by others in an effort to clarify this peculiar nature of Vi antigen.Along this line, we also made some experiments chiefly concerning with (1) the effect of Vi antigen on O hemagglutinability of some other species than salmonella typhosa, namely, some salmonella group except typhosa, some streptococci and klebsiella, (2) the heating effect of suspension of both V and W form salmonella typhosa (Ty2 and H901, respectively) on Vi and O hemagglutinability and (3) the heating effect of suspension of some salmonella group on O hemagglutinability.The followings are the summary of this report:(1) a. O hemagglutinability of the supernatant from the suspension (heated 1 hour at 100°C) of salmonella group such as, S. paratyphi A, S. paratyphi B, S. thompson, S. newport, and S. anatum was completely inhibited by Vi antigen (from ballerup) heated 1 hour at 60°C. (Sheep erythrocytes were used in this series)b. O hemagglutinability of klebsiella K and O using the supernatant from the suspension heated 1 hour at 100°C was completely inhibited by the above Vi antigen, except the case of O 4 antigen in which the suppression was not complete.c. Hemagglutinability of streptococcus sanguis I, II, streptococcus salivarius I, II, or K group streptococcus (using the supernetant of bacterial suspension heated 1 hour at 120°C) was also suppressed by the above Vi antigen. However, in the case of H group streptococcus or streptococcus MG the suppression was not complete.(2) a. The supernatants from suspension of salmonella typhosa Ty2 heated 1 hour at 58°C, 70°C, 80°C, 90°C and 100°C, respectively, showed high Vi hemagglutinability, but O hemagglutinabitity was completely inhibited in the case of heating at 58°C and 70°C. The others showed O hemagglutinability, though considerably weak. This could be interpreted as effect of rising temperature to weaken Vi antigen and increase of free O antigen.b. On the contrary, the supernatant of H901 strain showed no Vi hemagglutinability. O hemagglutinability was completely suppressed by the supernatant heated at 58°C, whereas, O hemagglutinability was seen high in the case of heating at 70°C and highest in the case of heating at 80°C-100°C.(3) Like the supernatant of H901 strain, the supernatant from suspension of salmonella paratyphi A, tompson, newport and anatum heated at 56°C-60°C showed suppression on O hemagglutinability gous antiserum. This property was destroyed at 100°C or by preserving it for several months. But only paratyphi B behaved somewhat different.