Thioredoxins are proteins, typically with a molecular mass of 12 kDa, that are widely, if not universally, distributed in the animal, plant, and bacterial kingdoms. Thioredoxins undergo reversible redox change through a disulfide group (S-S-->2 SH). Two cellular reductants--reduced ferredoxin and NADPH--supply the equivalents for reduction via different enzymes. The nature of the reductant serves as a basis for distinguishing and naming the two thioredoxin systems, which are discussed below in relation to their possible application in technology and medicine. Most of the discussion is referenced by general reviews. In the section dealing with animal cells, however, much of the material is quite recent. Thus, there, and elsewhere to a lesser extent, previously uncited studies are assigned specific references.