Heart sections from 16 mongrel dogs, two normal controls and 14 infected with Trypanosoma cruzi, were submitted to immunohistochemical staining with either rabbit anti-cow S100 Protein monoclonal antibody or rabbit anti-T. cruzi purified specific antibody, using the peroxidase technique to investigate the participation of the interstitial dendritic cells of the heart (IDCs) in myocarditis of Chagas disease. Trypanosoma cruzi antigens were revealed as granular and dense deposits in IDC membrane in the heart of infected dogs both during acute and chronic myocarditis, but not in normal controls. Anti-S100 Protein labeled the IDCs, both in normal and infected dogs and a significant increase in the numbers of IDCs occurred in the myocardium, proportionally to the intensity of the inflammatory infiltration. These findings suggest that IDCs, probably by presenting T. cruzi antigens to immune-competent cells, play an important role in the pathogenesis of Chagas disease.