Thioredoxin is a redox protein that is important for a variety of intracellular functions, possibly including regulation of transcription factor activity. We have shown that human thioredoxin has the same predicted amino acid sequence as adult T-cell-derived leukemic cell growth factor. Recombinant human thioredoxin stimulates the proliferation of Swiss murine 3T3 fibroblasts with an EC50 of 100 nM and the proliferation of a number of human cancer cells. Site-directed mutagenesis of the active-site cysteines of thioredoxin has shown that redox activity is necessary for the stimulation of cell proliferation. Added 125I-thioredoxin is taken up by cells in culture and could have intracellular action. A series of alkyl 2-imidazolyl disulfides have been shown to be competitive inhibitors of human thioredoxin reductase with Ki values of 3.3 to 8.6 microM. The compounds inhibited Swiss 3T3 serum-dependent proliferation with IC50 values of 2.0 to 4.0 microM, and one compound inhibited Swiss 3T3 thioredoxin-dependent proliferation with an IC50 value of 60 nM.