Abstract: Myoglobin is found exclusively in striated muscle. It can be considered a fetal antigen. Myoglobin was prepared from normal human thoracic muscle and was used to immunize the rabbits to obtain antimyoglobin antisera. The indirecf immunoperoxidase reaction was performed on numerous normal tissues and a variety of tumors. All the tissues were paraffin-embedded and formalin-fixed. The resuits from skeletal and cardiac muscle and rhabdomyosa-rcoma were positive. Of six cases of rhabdomyosarcoma, five were positive; one was undefined. Therefore, myoglobin should be considered as a specific marker for striated muscle and rhabdomyosarcoma, and the immunoperoxidase technique may be a useful method for the diagnosis of rhabdomyosarcom and other neoplasamas with rhabdomyoblastic differentiation.