Thioredoxin, a small redox protein with an active site disulfide/dithiol, is ubiquitous in bacteria, plants, and animals and functions as a reducing agent and modulator of enzyme activity. A thioredoxin has been purified to electrophoretic homogeneity from the leaves of Arabidopsis thaliana using procedures such as DE-52 ion exchange chromatography, Sephadex G-50 gel filtration, Q-Sepharose ion exchange chromatography, and DEAE-Sephadex A-25 chromatography. The purified thioredoxin was determined to be a single band on SDS-PAGE, and its moleculr weight was estimated to be 21 KDa, which was much larger than those of most other known thioredoxins. It was proved to be an f-type thioredoxin, since it could activate fructose-1, 6-bisphosphatase, but it could not activate NADP+-malate dehydrogenase. As a protein disulfide reductase, it could reduce the disulfide bonds contained in insulin. As a substrate, it showed a Km value of 20.2 ulcornerM on Escherichia coil thioredoxin reductase, and it had an optimal pH of 8.0. The molecular weight of the purified f-type thioredoxin is not consistent with those of the five divergent h-type thioredoxins already identified by cDNA cloning. The purified f-type thioredoxin is the first example isolate from A. thaliana.