Pleuropneumoma-like organisms (PPLO) of the catarrhal type were isolated from the brain of a Swiss mouse during the cranial passage of mouse hepatitis virus-MHV(C). Cranial injection of the PPLO alone in Swiss and Princeton weanlings was attended by a meagre growth of the organisms in the brain, with no pathologic change.

The growth of both catarrhal and conjunctival strains of PPLO in the brains of Swiss mice was greatly enhanced by the simultaneous injection of MHV(C). Rolling was not a characteristic sign prior to autopsy. Brain sections regularly showed a vigorous leukocytic response, commonly accompanied by the destruction of nerve cells in the anterior horns of the cerebrum. Injected in Princeton mice together with the virus, the organisms barely survived and were inactive. MHV(Pr) enhanced the growth and pathogenicity of PPLO in the brains of Princeton mice but failed to do so in Swiss.

The behavior of PPLO in the brain was likewise affected by the presence of agar, as earlier observed by Findlay et al. In comparison with the effect of MHV, the enhancement was reduced in rate in both strains of mice and was not accompanied by outward signs of nervous disorder. Hydrocephalus which often followed injection of the PPLO-agar mixture was also produced by agar-bouillon alone.

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