A previously undescribed disease of mice, characterized by acute interstitial myocarditis, and by periarterial pulmonary lesions, is reported.

The transmissibility of this disease has been shown, but a large proportion of inoculated mice appears to be resistant.

Microorganisms, rickettsial in form but not positively identified as such, are consistently present in the lesions. They are found predominantly in the cytoplasm of polymorphonuclear leucocytes.

Filtrates of fecal material from normal mice of the Harvard breeding stock may produce lesions in myocardium and pulmonary arteries indistinguishable from those occurring in this disease. Rickettsia-like bodies are also found in these lesions.

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