The administration of a single small dose of bacterial lipopolysaccharide produces in mice a considerable rise in properdin levels. This is accompanied by an early, transient, non-specific increase in resistance to certain bacterial infections. Bacterial lipopolysaccharides were shown to possess far greater activity than other substances previously studied in bringing about an elevation of properdin levels. After the injection of bacterial lipopolysaccharides, high molecular weight substances appear in the circulation, which interfere with the combination of properdin with zymosan and thus affect the assay of properdin. The administration of small amounts of bacterial lipopolysaccharides to mice at appropriate times before experimental infection "conditions" the mice so that they maintain normal or elevated properdin titers during the infectious process in contrast to control mice which show a progressive decline in properdin to low levels and death. The significance of this observation and its relationship to natural resistance to Gram-negative pathogens are considered.
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