This paper reports the purification and the properties of a thioredoxin from the fungus Aspergillus nidulans. This thioredoxin is an acidic protein which exhibits an unusual fluorescence emission spectrum. characterized by a high contribution of tyrosine residues. Thioredoxin from A . nidulans cannot serve as a substrate for Escherichia coli thioredoxin reductase. Corn NADP-malate dehydrogenase is activated by this thioredoxin in the presence of dithiothreitol, while fructose-I ,6-bisphosphatase is not. The amino acid sequence of Aspergillus thioredoxin was determined by automated Edman degradation after cleavage with trypsin, SV, protease, chymotrypsin and cyanogen bromide. The masses of tryptic peptides were verified by plasma-desorption mass spectrometry. The mass of the protein was determined by electrospray mass spectrometry and shown to be in agreement with the calculated mass derived from the sequence ( M , = 11 564). Compared to thioredoxins from other sources, the protein from A . niduluns displays a maximal sequence similarity with that from yeast (45%).Thioredoxins are a group of small heat-stable proteins found in numerous organisms. They are structurally characterized by a redox active site exhibiting the well-conserved sequence -Trp-Cys-Gly-Pro-Cys- [I]. Thioredoxin was first identified in Escherichia coli where it can serve as a reducing agent for different reductases, involved in the reduction of ribonucleotide [2], sulfate [3] and methionine sulfoxide [4]. The oxidized form of thioredoxin is reduced by a specific NADPH-dependent reductase [5].In higher plants, thioredoxins are involved in the modulation ofenzymic activities by thiol redox control, during light: dark transitions [6]. Three different thioredoxins have been purified from spinach leaves and characterized; two, located in the chloroplast, regulate the activity of target enzymes such as NADP-malate dehydrogenase, (m-type thioredoxin) and fmctose-l,6-bisphosphatase (f-type thioredoxin) [7], by reducing disulfide bonds. These chloroplast thioredoxins are reduced by a specific ferredoxin-thioredoxin reductase [8]. The third thioredoxin, called thioredoxin h [9] was purified from other compartments of the cell (mitochondria, cytosol, endoplasmic reticulum) [lo]. It has been assigned a physiological function only in seeds, where it reduces cystine-rich proteins, such as purothionin, a-amylase and trypsin inhibitors [ll].