ADF (actin depolymerizing factor) is an M(r) 19,000 actin-binding protein present in many vertebrate tissues and particularly abundant in neuronal cells. We have cloned human ADF and here show it to be identical in sequence to porcine destrin. Human ADF expressed in Escherichia coli behaves like native ADF from porcine brain. It binds to G-actin at pH 8 with a 1:1 stoichiometry and Kd approximately 0.2 microM, thereby sequestering monomers and preventing polymerization. It does not cosediment with F-actin at this pH, but severs actin filaments in a calcium-insensitive manner. The severing activity is only about 0.1% efficient. By contrast, at pH values below 7, ADF binds to actin filaments in a highly cooperative manner and at a 1:1 ratio to filament subunits. When the pH is raised to 8.0, the decorated filaments are rapidly severed and depolymerized.