Using rabbit anti-rabbit heart auto-antibodies, cultures of dissociated beating rat heart cells were examined for the presence of heart auto-antigens by immunofluorescence. Several patterns of staining of myocardial cells were observed. One type of auto-antibody revealed crisp myofibrillar cross-striations, consisting of wide bright segments often split by a thin dark band. Auto-antibody producing this pattern was potent for auto-antigen B. Rabbit anti-rabbit heart auto-antibodies rich in the auto-antigen D systems produced a granular staining in the myocardial cells, while an auto-antibody reactive with all five of the cardiac auto-antigens showed both granular and myofibrillar cross-striations. All the heart auto-antibody preparations reacted with cultured neonatal rat skeletal muscle cells to show diffuse cytoplasmic fluorescence and fine myofibrils. In all cases, rabbit anti-rabbit heart auto-antibodies failed to stain significantly the non-contractile cells present in the tissue cultures (‘endotheliod’, fibroblast, etc.). Sarcolemmal staining was not a prominent feature of these reactions. When the rabbit anti-rabbit heart auto-antibodies were exposed to living beating rat heart cells, no staining was observed, indicating the absence of these antigens on the cell surf
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