Smads transduce intracellular signals initiated by members of the transforming growth factor beta (TGF beta) family, including activins, TGF betas, and bone morphogenetic proteins. Recently, various models concerning the mechanism of Smad action have been proposed; however, these models are basically qualitative. Quantitative verification of the validity of the models requires significant amounts of purified Smad proteins, but purification of full-length Smad protein has not been straightforward even using recombinant protein expression systems. Here, we report purification of Smad proteins expressed in E. coil as glutathione S-transferase-fused proteins. By glutathione-Sepharose affinity purification, ATP treatment, DEAE-Sepharose and hydroxylapatite columns, expressed Smads were purified to near homogeneity as judged by SDS-PAGE; protein recovery was ca. 1 mg/l culture for Smad2 and 100 mug/l culture for Smad4. The purified Smad proteins had three known in vitro activities: Smad2 phosphorylation by TG;FP receptor complexes immunoprecipitated from COS7 cells, Smad4 binding to Smad-binding DNA element, and Smads interaction with calmodulin. The data suggest that purified proteins could be useful for biochemical analyses to evaluate the current models quantitatively.