Thioredoxins are low molecular weight proteins which serve as hydrogen donors in a wide variety of redox reactions via reversible formation of a disulfide bridge between two neighboring cysteins. We present data demonstrating that in Dictyostelium discoideum thioredoxins constitute a highly conserved multigene family. We have isolated cDNA clones coding for three different Dictyostelium thioredoxins which show 80% mutual identity. Analysis of genomic Southern blots suggests the presence of additional genes. Except for the active site (Trp-Cys-Gly-Pro-Cys), there are only a few amino acid identities with thioredoxins from other organisms. Identity scores do not exceed 43%, the value found with the human lymphocyte protein. DdTRX1 was expressed in Escherichia coli, purified, and shown to have thioredoxin activity, as judged by its capacity to activate the NADP-malate dehydrogenase. Due to its life cycle, during which individual amoebae form a multicellular fruiting body, Dictyostelium is used to study developmental processes such as cell-type differentiation and regulation of gene expression. Transcript levels of Dictyostelium thioredoxins were regulated during the developmental cycle. Low levels of mRNAs could be detected during growth. After the onset of development, where essentially no cell divisions take place, message levels increased with maximal expression during aggregation. In later multicellular stages, RNA levels declined again. The same expression pattern could be seen for all cloned thioredoxins. Protein levels paralleled this time course with a delay of several hours as judged by Western blot and activity measurements.