Histopathology in Rockland mice immunized against experimental American trypanosomiasis (Chagas' disease) was investigated. The antigen was prepared from culture forms of Trypanosoma cruzi disrupted by pressure. The dose used to immunize the animals were 50, 75, and 150 mg. Immunized mice killed but not challenged by a virulent strain of T. cruzi did not show fundamental alterations, in comparison with the normal control mice. Several groups of immunized mice were challenged with 50 LD50 doses of virulent T. cruzi. The mice that were immunized with three doses of 25 mg, and those immunized with three doses of 50 mg showed cellular infiltration in different organs on the 60th day after challenge. This infiltration has disappeared completely on the 120th day. It was not possible to find nests of Leishmania-like bodies in either of these latter groups.