Two carcinomas of the breast containing large areas of sarcomatous tissue were studied by light and electron microscopy. In one of these, the sarcomatous element was frankly cartilaginous and in the other, predominantly myxoid but with small cartilaginous-looking foci. By light microscopy, a highly suggestive metaplastic transition could be traced from cells within the epithelial nests to those within the sarcomatous lobules. Ultrastructurally, cells in the former region showed epithelial characteristics and those in the latter region, mesenchymal and/or cartilaginous features. The carcinomatous cells contained desmosomes and formed intercellular spaces lined by microvilli; a few cells showed prominent profiles of rough endoplasmic reticulum. In the first case, the cells in the immediate vicinity of the epithelial nests and those in the fully developed cartilaginous regions showed a progressive dilatation of their endoplasmic reticulum to form large sac-like structures filled with a finely granular and floccular material. The intercellular matrix was electron lucent and contained scattered dense particles, fibrillo-granular material and collagen fibres. Condensation of this material at some distance from the cell resulted in the formation of lacunae. In the second case, the cells in the myxoid areas also showed prominent dilatation of endoplasmic reticulum.