This investigation was designed to study the origin of the stromal component of human mammary carcinoma. Tissues from 30 proved breast carcinomas were studied. Under a light microscope with special stains, an increased connective tissue component was seen in carcinoma. Gluteraldehyde-fixed tissue was examined under an electron microscope. The stroma appeared to be composed of fibroblastic, myofibroblastic, histiocytic and primitive mesenchymal cells. Elastic fibers were intimately intermingled with collagen and reticulin and seen in close association with carcinoma cells. In some areas they were seen inside the carcinoma cells with discontinuous cytoplasmic membrane, as if they were formed by the cancer cells. This observation suggests that breast carcinoma cells play an important role in the production of elastic fibers in association with stromal cells and also stimulate the proliferation of the stromal cellular component.