When mice were inoculated orally with 10⁰-10⁴ cells of mouse-passage descendants of Salmonella typhimurium derived from a carrier-dog feces, the large-intestinal descendant showed the smallest ID_&lt50&gt value and was followed by the descendants of the mesenteric lymph node, liver and spleen. A descendant giving a smaller ID_&lt50&gt value yielded a large number of colonies having hemagglutinating activity on the blood agar plate. From the colonies, 7 fimbriate and 3 non-fimbriate descendants were selected. The fimbriate descendants having 19 to 91% of fimbriate cells showed various degrees of agglutination with 8 kinds of erythrocytes. The non-fimbriate destendants did not agglutinate the given erythrocytes. There was a tendency among the fimbriate descendants that a descendant with a large number of fimbriate cells gave a smaller ID_&lt50&gt value by oral administration against mice (10¹-10⁴ cells/mouse). The non-fimbriate descendants were capable of infecting mice rarely by oral administration with 10¹-10⁴ cells/mouse. On the contrary, both fimbriate and non-fimbriate descendants gave almost the same LD_&lt50&gt value by intraperitoneal inoculation. From statistical analysis on ID_&lt50&gt value, it was shown that the infectivity of the large-intestinal descendant was higher than that of a fimbriate descendant with 91% of fimbriate cells isolated from the intestine. The descendants of the mesenteric lymph node and liver were higher in infectivity than any other fimbriate descendant with 50 to 77% of fimbriate cells.