A bilateral breast tumor in an 11-year-old girl was diagnosed as a malignant mesenchymal neoplasm by fine needle aspiration biopsy, and its muscular origin was suggested. An immunoperoxidase stain for the demonstration of intracellular myoglobin in destained cytologic aspirates exhibited a positive reaction of variable intensity in some of the undifferentiated round cells and in the more mature bipolar sarcomatous elements. These findings were confirmed in the histologic sections of the excised tissue. Needle aspiration is helpful in these rare cases of primary breast rhabdomyosarcoma, which often present with a solid, undifferentiated histologic pattern in this age group. In addition to the cytologic characteristics, the possibility of detecting muscle antigens as markers for these embryonal small cells, even in previously stained slides, provides a successful method for defining the specific type of sarcoma.