A new connective tissue protein, fibrillin, has been isolated from the medium of human fibroblast cell cultures. It has a molecular mass of approximately 350,000 D and contains intrachain disulfide bonds. It is glycosylated, stains with periodic acid-Schiff reagent, and contains [³H] glucosamine but not [³⁵S] sulfate. It is resistant to digestion by bacterial collagenase. Using monoclonal antibodies (mAbs) specific for fibrillin, its widespread distribution in connective tissues such as skin, lung, kidney, vasculature, cartilage, tendon, muscle, cornea, and ciliary zonule was demonstrated. Electron microscopic immunolocalization showed fibrillin to be associated with extracellular structural elements called microfibrils, specifically 10-nm microfibrils. These microfibrils are found in most tissues and are associated with basement membranes and elastic fibers. They are distinct from 3–5-nm microfilaments, which are connected to proteoglycan granules. Fibrillin is arrayed periodically along microfibrils and may be aligned within bundles. The periodicity of the epitope matches the interstitial collagen band periodicity. In contrast, type VI collagen is associated with small diameter microfilaments interwoven with large banded collagen fibers but not with microfibrils containing fibrillin. Fibrillin is a major structural component of microfibrils and is resistant to collagenase digestion. It is a glycoprotein with a molecular mass of approximately 350,000 D and is present in various connective tissues. Immunofluorescence studies showed fibrillin in human skin, kidney, lung, muscle, tendon, cornea, and placenta. Electron microscopic immunolocalization confirmed fibrillin's association with microfibrils in human skin and other tissues. Fibrillin is a component of microfibrils in elastic tissues such as aorta and ear cartilage, as well as in elastin-poor tissues like cornea, tendon, and ciliary zonule. It is also present in alveolar connective tissue, kidney glomeruli, and muscle sheaths. The periodicity of fibrillin along microfibrils suggests an ordered aggregation into microfibrils and bundles. Fibrillin is a collagenase-resistant, nonsulfated glycoprotein with a molecular mass of approximately 350,000 D. It is a major structural component of microfibrils and is present in various connective tissues. The study provides evidence that fibrillin is a key structural component of microfibrils in connective tissues.A new connective tissue protein, fibrillin, has been isolated from the medium of human fibroblast cell cultures. It has a molecular mass of approximately 350,000 D and contains intrachain disulfide bonds. It is glycosylated, stains with periodic acid-Schiff reagent, and contains [³H] glucosamine but not [³⁵S] sulfate. It is resistant to digestion by bacterial collagenase. Using monoclonal antibodies (mAbs) specific for fibrillin, its widespread distribution in connective tissues such as skin, lung, kidney, vasculature, cartilage, tendon, muscle, cornea, and ciliary zonule was demonstrated. Electron microscopic immunolocalization showed fibrillin to be associated with extracellular structural elements called microfibrils, specifically 10-nm microfibrils. These microfibrils are found in most tissues and are associated with basement membranes and elastic fibers. They are distinct from 3–5-nm microfilaments, which are connected to proteoglycan granules. Fibrillin is arrayed periodically along microfibrils and may be aligned within bundles. The periodicity of the epitope matches the interstitial collagen band periodicity. In contrast, type VI collagen is associated with small diameter microfilaments interwoven with large banded collagen fibers but not with microfibrils containing fibrillin. Fibrillin is a major structural component of microfibrils and is resistant to collagenase digestion. It is a glycoprotein with a molecular mass of approximately 350,000 D and is present in various connective tissues. Immunofluorescence studies showed fibrillin in human skin, kidney, lung, muscle, tendon, cornea, and placenta. Electron microscopic immunolocalization confirmed fibrillin's association with microfibrils in human skin and other tissues. Fibrillin is a component of microfibrils in elastic tissues such as aorta and ear cartilage, as well as in elastin-poor tissues like cornea, tendon, and ciliary zonule. It is also present in alveolar connective tissue, kidney glomeruli, and muscle sheaths. The periodicity of fibrillin along microfibrils suggests an ordered aggregation into microfibrils and bundles. Fibrillin is a collagenase-resistant, nonsulfated glycoprotein with a molecular mass of approximately 350,000 D. It is a major structural component of microfibrils and is present in various connective tissues. The study provides evidence that fibrillin is a key structural component of microfibrils in connective tissues.