The acute phase of arteritis of the common iliac artery induced in young rats by the inoculation of viable Erysipelothrix rhusiopathiae (E. rhusiopathiae) was examined. Within three days post inoculation, cell infiltration was first detected in the periarterial tissue, extending centripetally to the outer two-thirds of the media. However, the inner third of the media and the intima were free from cell infiltration, and the media showed no degeneration. E. rhusiopathiae first invaded the periarterial tissue and the adventitia, thereafter, invaded the outer two-thirds of the media, but not the inner third of the media or the intima. Invasion by the bacteria preceded cell infiltration. Arteritis was not induced by heat killed bacteria or the supernate from the culture medium. The arteritis was produced preferentially in large or medium-sized muscular arteries. Immunofluorescence test by FITC-conjugated peanut lectin demonstrated the desialated sites by the effect of the neuraminidase produced by E. rhusiopathiae in the arterial region that were concomitant with the invasion of the bacteria. C3 was also detected along the adventitia of the artery. The results demonstrated that viable E. rhusiopathiae can induce arteritis of muscular arteries and that the neuraminidase produced by the bacteria may play a role in the pathogenesis.