Fibrillin-rich microfibrils are important structural elements widespread throughout connective tissues. Genetic defects identified in the Ca2+ binding sites of fibrillin have severe effects and in addition Ca2+ has a marked effect on the microfibrillar structure. We have studied the role of Ca2+ on the mechanical behavior of fibrillin-rich microfibrils using the micro-needle technique. We find that Ca2+-depletion results in a 50% decrease in rest length and reduces the stiffness of fibrillin-rich microfibrils. At high strain, irreversible damage occurs. This behavior is consistent with Ca2+ stabilization of interactions between consecutive EGF-like domains and breakdown in the quaternary structure upon over-extension. Proteins 2001;45:90–95. © 2001 Wiley-Liss, Inc.