Studies conducted on rats have shown an increased susceptibility after splenectomy to intravenously introduced pneumococci. One of the main obstacles in such experiments, as regards reproducibility, is the maintenance and handling of the bacteria. A procedure is described for the preparation of standardized inocula and preservation of the bacteria at -80 °C. No change in viability or virulence was observed after storage for at least 6 months. A pronounced difference in susceptibility to type 1 pneumococci was found between sham-operated (LD100 > 2 × 104 colony-forming units) and splenectomized rats (LD100 2 colony-forming units); in the latter group, the survival time was inversely correlated to the quantity of bacteria injected.