The stroma in infiltrating breast carcinomas, with particular reference to stromal spindle cells, has been studied by electron microscopy. A mixture of cells including resting fibroblasts, active fibroblasts, early myofibroblasts, and mature myofibroblasts has been identified. In loose stroma, myofibroblasts possessed prominent organelles and showed secretory products along the cell surface, whereas in dense stroma, there was relative prominence of cytoplasmic filaments as well as other features consistent with a contractile state of myofibroblasts. The degree of myofibroblastic proliferation was related to the growth pattern of the tumour. It is suggested that the infiltrating process of cancer cells is analogous to wound production and healing with continuous granulation tissue and scar formation resulting in the characteristic desmoplastic reaction seen in certain breast carcinomas.