Immunohistochemical localization of myoglobin in nonmuscular cells.

Myoglobin has been detected by the immunoperoxidase technique in the cytoplasm of nonmuscular cells present in malignant tumors infiltrating skeletal muscle. The positive cells were reactive histiocytes in one case, and malignant cells (breast carcinoma, malignant melanoma, and malignant lymphoma) in the others. These findings indicate the need to exercise caution in the interpretation of myoglobin stain for the diagnosis of rhabdomyosarcoma under these circumstances.