As one aspect of research to clarify the defence mechanism in enteric infection, comparison of phagocytosis to adherent cells from peritoneal cavity (macrophages) and infectivity to HeLa cells between Shigella flexneri 2b 17-A (a virulent strain) and Shigella flexneri 2b 17-N (an avirulent strain) was carried out. The findings of the experiments may be summarized as follows:1) Bacillus suspensions of both strains were contacted with macrophages or HeLa cells at 37°C for 1 or 2 hours. After separating bacillus by washing, these cells were incubated further for the confirmation of their phagocytic or infection rates. Phagocytic rates of the virulent strain or the avirulent one in macrophages reached about 20% in 0-2 hours after incubation. The rate of the former increased with the lapse of incubation time but that of the latter trended to decrease. HeLa cells were infected with the virulent strain but not with the avirulent strain.2) In the case of phagocytosis of macrophages with larger contact dose of bacteria or longer contact time, initial phagocytic rates (at the incubation on 0 hr) were increased and destractive degree of macrophages were enlarged. So, there could hardly be found any difference on an initial phagocytic rate and destructive degree of macrophages between the virulent strain and the avirulent one.3) Macrophages were contacted by both strains treated with 70% alcohol for 30 minutes or 100°C for 30 minutes, thereafter they were incubated further. Initial phagocytic rates of them showed about 20% and then they decreased along with the incubation time. Eight hours after incubation, there were not found bacteria in any macrophage. The same experiment was carried out with HeLa cells, and bacteria were not found in HeLa cells at any time of incubation.4) A morphological analysis of phagocytes was performed using transmission electron microscope. Immediately after inoculation of the virulent or the avirulent strain into the peritoneal cavity of guinea pigs, their phagocytes began to ingest them. In phagocytes from the guinea pigs 4 hours after the injection of the virulent strain, most of their phagocytic vacules were of a loosed type, but that in phagocytes from the animal injected with avirulent strain trended to be of a tight type.A limiting membrane was recognized around the bacilli in phagocytes from animal inoculated with both strains, irrespective of their virulence. On the contrary, the membrane did not found around the bacilli in HeLa cells.