A dose of 5 × 109S. typhi (strain Ty2) and S. paratyphi B (strain HB3) administered to white mice orally, caused an infection of various organs of the mouse, namely: spleen, liver, kidneys, lungs, gall-bladder, mesenteric lymph nodes, jejunum and of the blood stream. The percentage of infected spleens was higher than that of other organs.

With the above-mentioned dose of Ty2, the infection of the liver was found to be five times higher than that of the gall-bladder. The infection of the latter lasted for about 8–9 days, whereas that of the liver for 18–19 days.

The bacteria appeared in the various organs: liver, spleen, kidneys, and lungs in about 2 min. after oral infection, and in the blood stream after about 20 sec.

The speed and intensity of the invasion of micro-organisms from the gastrointestinal tract to the blood stream depends neither upon the volume of the dose (0·5 ml. or 0·1 ml.), nor upon the content of the mouse's stomach.

The M.I.D. for the blood stream of the mouse by oral infection is about 5 × 105 bacteria for Ty2, HB3 and AH6.

In blood samples collected by cardiac puncture 2–3 min. after oral infection, an average of 5 × 102 micro-organisms/ml. blood was found. The count of bacteria increased and reached its peak after 20 min. (about 3 × 103 bacteria). After that period, the number decreased: 1·6 × 103 after 1 hr., 8 × 102 after 2 hr. and 1·7 × 102 after 3 hr. In spite of this considerable decrease, the blood did not became sterile until after several days.

In the infected spleen (by oral infection) an inverse process was observed: at the beginning only a small number of micro-organisms was found (about 5 × 10/100 mg. spleen), but afterwards the number of bacteria increased (1·2 × 102 after 24 hr. and 5 × 102 after 4 days).

In mice infected by the subcutaneous route with a dose of 5 × 109 bacteria, the number of micro-organisms in the blood stream did not differ significantly from that found after oral infection, whereas infection by intraperitoneal route caused a severe infection. When 5 × 107 micro-organisms (LD50) were administered by intraperitoneal route, 107 bacteria appeared in the blood stream of the moribund mice while in the spleen about 4·1 × 107 were found.

The author wishes to express his appreciation to Prof. I. Gurewitch and to Dr R. Rozansky from the Department of Clinical Microbiology, the Hebrew University-Hadassah Medical School, Jerusalem, for their advice and criticism.