We have studied the mechanism of Ca++-dependent restriction of actin filament length by villin, one of the major actin-associated proteins of intestinal microvilli microfilament bundles. Villin acts, even at a ratio of 1 to 1000 with respect to actin, very efficiently as a Ca++-dependent nucleation factor on actin assembly. This gives rise to unidirectional assembly, with the morphologically defined "barbed" end of the resulting filament being capped. Consequently, at steady state treadmilling of actin monomers through the filament is inhibited. Increase of the villin-to-actin ratio enhances the number of nucleated filaments necessarily shorter in length. This results finally in nonsedimentable F actin and a low molecular weight complex of one villin and three monomeric actins, which itself is a potent nucleator. Thus restriction of actin assembly by villin is not due to a direct inhibition of assembly but arises as the consequence of strongly enhanced nucleation followed by unidirectional elongation at the pointed end of the nucleated filaments. In addition, in the presence of Ca++-villin, but not the villin-actin complex, seems able to "break" or "sever" preformed F actin filaments. Thus a variety of cellular phenomena-nucleation, unidirectional assembly, filament end capping, nonpolymerizable actin and F actin bundles-can be observed in vitro in a two-protein component system modulated by the concentration of free Ca++.