The growth and pathogenicity of were studied in chicken embryo tracheal organ cultures. In these cultures attained titres of 10-10 CCU per ml of Eagle's medium and there was inhibition of ciliary activity. The cilia-stopping effect (CSE) was not closely related to dose, as a 50 per cent. reduction of ciliary activity occurred at about the same time after inoculation of organisms differing in number by as much as 100- to 10,000-fold. The addition of neuraminidase or catalase to the cultures did not inhibit the CSE of in fact, the CSE was potentiated in cultures containing catalase. The CSE was present but delayed in organ cultures maintained in medium that contained , the continued multiplication of which was suppressed by tetracycline. Thus it seems that either the accumulation of toxic products in the medium contributes to tissue damage, or that the organisms themselves do so; but neither cytadsorption nor peroxide production appears to be important in the pathogenicity of .


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