The biochemical properties of villin purified from the brush borders of chicken and rat small intestines were compared, with emphasis on their physical properties and their Ca++-dependent interaction with actin. Like chicken villin, rat villin exists as two isoforms present in equimolar concentrations; the rat isoforms are slightly more acidic than those of chicken villin (6.08 and 6.11 versus 6.26 and 6.34). Rabbit antisera raised against either villin crossreacted with the other one. Like the avian protein, rat villin bundled F-actin at calcium concentrations below 0.1 microM. Above approximately 1 microM calcium, it accelerated the rate of actin assembly and restricted filament lengths of F-actin formed either during coassembly with villin or by addition of villin to preformed filaments. The threshold calcium concentration required for effective severing of preformed filaments was approximately tenfold higher than that required for restricting lengths during coassembly. The extent of filament shortening was proportional to the amount of villin present. At a fixed villin concentration, filament length decreased with increasing [Ca++] over a broad range from 10(-7)-10(-4) M. In general, the mean filament lengths and the dispersion about the mean value were lower in samples where filaments were coassembled with villin than when villin was added to preformed filaments.