Summary and Conclusions: Living staphylococci and paratyphoid bacilli injected intravenously into rabbits are quickly removed from the circulating blood stream and are localized particularly in the liver and spleen where they are ingested and destroyed by phagocytes. The lungs, bone marrow and omentum remove distinctly fewer numbers and such organs as the kidney, suprarenal, striated muscle, brain, testis and thyroid remove negligible numbers under comparable conditions. In other words, the primary localization occurs principally in the two organs containing many macrophages and a sinusoidal type of blood flow, whereas practically no localization occurs in organs poorly supplied with macrophages and with a rapid blood flow through vessels lined with ordinary endothelium. Active immunization does not significantly affect the comparative degrees of localization for different organs of the same animal, although it leads to a more energetic removal of the bacteria from the blood stream by the liver and spleen. Bacteria are concentrated more quickly in the liver and spleen of immune than of normal animals but are also killed more rapidly within these organs. Comparatively few living bacteria of these types are eliminated by the bile. Active intravenous immunization leads to a stimulation of mesenchymal tissues, particularly in the liver and spleen, with a resulting elevation of the functional state of the system of macrophages.