Studies presented in this report are concerned with the effect of endotoxin upon the release of bactericidal antibodies as determined by the agar plaque technique. We found that shortly after the injection of mice with small amounts of endotoxin the number of plaque-forming cells in the spleen was substantially reduced and remained low for about 12 hr. At the same time the titer of specific bactericidal antibodies in the circulation was increased, 6-mercaptopurine was shown to be an effective suppressor of the immune response by inhibiting the multiplication of bactericidin-forming cells. However, administration of this drug did not modify the initial responses of antibodies and plaque-forming cells to endotoxin. Thus, we postulate that endotoxin does not induce destruction and subsequent reconstitution of bactericidin-forming cells, but that the observed changes represent a release of preformed antibodies from competent cells. For a limited time, thereafter, these cells become inactive, but soon resume their normal function of releasing bactericidal antibodies.

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