We produced and characterized a specific monoclonal antibody (mAB) designated GR-ICOR-2. This mAb recognizes sarcomeric actin molecules (43 kDa) and was used in an immunohistochemical analysis of staining patterns in Hamburger and Hamilton's stages 18, 22 and 25 (HH 18, 22 and 25) embryonic chick hearts. Staining showed a mainly cytoplasmic distrubition in three regions: the atrioventricular (AV) canal cushion tissue, the primitive ventricle, and conal crests. In addition, this mAb-cross-reacted with rabbit and human cardiac and skeletal muscle tissue; but not with smooth muscle tissue.