Over 2 million silicone breast prostheses have been implanted since they were introduced in the 1960s. After implantation, a fibrovascular tissue reaction referred to as a "capsule" is observed. Many consider this capsule to be a static structure, an effective barrier to the egress of foreign material. However, reports documenting the presence of silicone within lymph nodes of patients with apparently unruptured implants indicate that silicone may be transported away from the breast-implant capsule. To characterize the cells making up the breast-implant capsule, 183 capsules from 103 ruptured or bleeding implants and 80 intact implants were studied. Gross and light microscopic studies were performed on all, and selected capsules were subjected to ultrastructural study and Fourier-transform infrared spectroscopy. Light microscopic examination of the capsule revealed an organized, layered structure with an associated network of endothelia-lined spaces. The capsules varied in cellularity, depending on the type and integrity of the implant. The superficial cell layer of all capsules had cytoplasmic processes directed toward the surface. These long cytoplasmic processes contained vacuoles ultrastructurally, indicating phagocytic and pinocytotic capability. These cells bore immunological markers of bone marrow derived macrophage-type cells. The extracellular matrix of the surface layer consisted of an amorphous fibrillar protein lacking the ultrastructural periodicity of mature collagen. No cell-to-cell junctions were observed. Deeper capsular layers were characterized by fibroblast-type cells in a collagen matrix. No capsules studied contained basement membrane or basal lamina between the stroma of the capsule and the surface cells.(ABSTRACT TRUNCATED AT 250 WORDS)