Up to now, the cellular localization pattern of monoclonal antimyosin antibodies (AMA) during acute rejection has not been described. Focused on this the authors made immunohistochemical and scintigraphic studies (AMS) with AMA in an animal transplantation model. Heterotopic cervical heart transplantation was performed in 12 mongrel dogs. Immunosuppression consisted of triple drug therapy. As standard the grafts were examined by daily transmural biopsies and routine histology. Dependent on the daily biopsy results, 0.5 mg of indium 111 ((111)In)-labeled AMA-Fab was injected. Subsequently every 2 hours transmural biopsy cylinders were taken out of the right ventricle and examined in indirect peroxidase staining technique. Forty-eight hours after AMA injection, scintigraphy in single photon emission computed tomography (SPECT) technique (AMS) was carried out and the heart-to-lung ratio (H/L-ratio) was calculated. The immunohistochemical maximum of AMA accumulation could be found 20 to 72 hours after AMA injection. This means that a scintigraphic examination should be done earlier than 20 hours and later than 3 days after injection. Dependent on the grades of bioptic rejection diagnosis a specific morphologic AMA localization was seen (grade I+II intercellular and slightly intracellular detection of AMA, grade III strongly intracellular and in particular perinuclear accumulation of the antibody, p<0.01). Moreover, the authors found a good correlation between scintigraphic H/L-ratio results and the corresponding histologic findings (grade I: H/L = 2.1 +/- 0.2; grade II: H/L = 3.1 +/- 0.2; grade III: H/L = 3.5 +/- 0.3; n = 19; p<0.02). The recently described positive AMS scans even in cases of mild rejection seem to be subject to an intercellular AMA localization. This typical AMA morphology during mild rejection favors the theory of the pore-forming protein allowing the efflux of myosin fragments as effector mechanism of cytotoxic lymphocytes in the early phase of acute rejection. The immunohistochemical AMA examination could explain the present discrepancy between positive AMS results of an intracellular protein in cases of mild or moderate acute rejection when visible cellular damage in the corresponding routine histology is absent.