The reactivity of human thioredoxin (HTR) was tested in several reactions. HTR was as efficient as E. coli or plant and algal thioredoxins when assayed with E. coli ribonucleotide reductase or for the reduction of insulin. On the other hand, HTR was poorly reduced by NADPH and the E. coli flavoenzyme NADPH thioredoxin reductase as monitored in the DTNB reduction test. When reduced with dithiothreitol (DTT), HTR was much less efficient than thioredoxin m and thioredoxin f, the respective specific thioredoxins for the chloroplast enzymes NADP-malate dehydrogenase (NADP-MDH) and fructose 1,6 bisphosphatase (FBPase). Finally, HTR could be used in the photoactivation of NADP-MDH although less efficiently than thioredoxin m, proving nevertheless that it can be reduced by the iron sulfur enzyme ferredoxin thioredoxin reductase in the presence of photoreduced ferredoxin. Based on sequence comparisons, it was expected that HTR would display a reactivity similar to chloroplast thioredoxin f rather than to thioredoxin m. However the observed behavior of FTR did not exactly fit this prediction. The results are discussed in relation to the structural data available for the proteins.