The acute phase of the arteritis of the common iliac artery induced in young rats by the inoculation of viable Erysipelothrix rhusiopathiae was examined. Arteritis of the common iliac artery was observed within 4 days after inoculation in every rat that was inoculated with the live bacteria. Within 2 days after the inoculation, infiltration by mononuclear and polymorphonuclear cells was first detected in the periarterial tissue extending centripetally to the outer two-thirds of the media. Invasion by the bacteria always preceded cell infiltration. Immunofluorescence test by FITC-labeled peanut lectin demonstrated the desialated sites by the effect of neuraminidase produced by the bacteria in the arterial wall concomitantly with the invasion of the bacteria. Linear presentation of C3 along the external elastic lamina of that artery was also detected. The results showed that viable E. rhusiopathiae could induce arteritis of muscular arteries and that the neuraminidase produced by the bacteria might play a role in the progress of arterial inflammation. We consider that this model will contribute to clarifying the progression of the arteritis process, and to assessing the efficacy of antiinflammatory drugs to inhibit the arterial inflammation.