Gelsolin regulates the dynamic assembly and disassembly of the actin-based cytoskeleton in non-muscle cells and clears the circulation of filaments released following cell death. Gelsolin is a six-domain (G1-G6) protein activated by calcium via a multistep process that involves unfolding from a compact form to a more open form in which the three actin-binding sites (on the G1, G2, and G4 subdomains) become exposed. To follow the global structural changes that accompany calcium activation of gelsolin, small-angle x-ray scattering (SAXS) data were collected for full-length human plasma gelsolin at nanomolar to millimolar concentrations of free Ca 2؉ . Analysis of these data showed that, upon increasing free Ca 2؉ levels, the radius of gyration (R g ) increased nearly 12 Å , from 31. , even higher calcium concentrations help to stabilize a more open structure, with increases in R g and D max of ϳ2 and ϳ15 Å , respectively. At these higher calcium levels, the SAXS-based models provide a molecular shape that is compatible with that of the crystal structures solved for Ca 2؉ /gelsolin C-terminal and N-terminal halves ؎ monomeric G-actin. Placement of these crystal structures within the boundaries of the SAXS-based model suggests a movement of the G1/G2 subunits that would be required upon binding to actin.