Fibrous tissue capsules around silicone-gel and saline filled breast implants were examined by light microscopy as well as by transmission and scanning electron microscopy. This study revealed the following details: 1. Regularly arranged dense connective tissue forms the basis of the reactively developed capsules around the breast implants. This tissue contains bundles of collagenous fibers which are densely packed and lie parallel to each other forming structures of great tensile strengthes. 2. The outer surface of the connective tissue capsules contain reticulum fibers, the diameter of which is small. The fibers branch variously to form a netlike framework, which obviously can support the mechanical significance of the collagenous material. 3. At the inner surface fibrocytes and histiocytes are single layered and form an epithelium-like structure. These cells possess specializations of their free surface as microvilli, microplicae, and cell protrusions. In some areas dense connective tissue forms a felt-like surface. 4. Contractile fibroblasts (myofibroblasts) were found in these fibrous capsules. These cells share electron microscopic features of both smooth muscle cells and fibroblasts. Along with the rough endoplasmic reticulum and Golgi apparatus, these cells have long thin bundles of microfilaments. 5. The connective tissue of the capsules contains also capillaries and fibrocytes, which are usually deployed along bundles of collagen fibers and appear in sections as fusiform elements with long processes. The fibrocytes also contain lysosomes, and because these are involved in hydrolysis or digestion of material within the cell, their appearance depends upon their functional state, leading to their characteristic pleomorphism. 6. The frequently resulting unshapelyness of the breast implants cannot be caused by the action of the so-called myofibroblasts, the number of which is too small, but more likely because of the unelestic arrangement of large amounts of collagenous material.