The mechanical properties of composite membranes obtained by self-assembly of actin filaments with giant fluid vesicles are studied by micromanipulation with optical tweezers. These complexes exhibit typical mechanical features of a solid shell, including a finite in-plane shear elastic modulus ( approximately 10(-6) N/m). A buckling instability is observed when a localized force of the order of 0.5 pN is applied perpendicular to the membrane plane. Although predicted for polymerized vesicles, this is the first evidence of such an instability.