Two distinct particles have been recognized in the extracellular fluid of B. pertussis cultures. Both appeared to arise from the surface (cell wall) of the organism.

One of these, a membranous particle, seemed to derive from long projections on the organism composed of the outer membrane of the cell wall.

The second particle, a fine filament, was not readily comparable with any previously described bacterial structure.

The two particles could be separated from each other by gradient centrifugation in CsCl.

Lymphocytosis-promoting factor and histamine-sensitizing activity were only associated with fractions containing the fine filaments.

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