Immunoglobulins (Igs) reacting against endothelial and vascular structures and striated muscle cells as well as against cells from a peripheral nerve were detected by indirect immunofluorescent test (IIF) in a rhesus monkey infected for 29 yr with Trypanosoma cruzi. Anti-T. cruzi antibodies in this monkey showed a titer of 1:128 in the IIF test and the direct agglutination test. Tissue-reacting Igs were bound in vivo to the tissues, as was established by direct treatment of a biopsy of the deltoid muscle with Ig-labeled antisera. Electron microscopy of this tissue showed that Igs reacted with the plasma membrane of the muscle cells. Neither tissue-reacting Igs nor specific antibodies were detected in three uninfected adult monkeys.