ADF (adult T-cell leukemia-derived factor), originally defined as an inducer of interleukin 2 receptor/alpha (IL-2R/alpha), is a homologue of thioredoxin. ADF is constitutively produced by human lymphoid cell lines transformed by human T-lymphotropic virus type I (HTLV-I) or Epstein-Barr virus (EBV). ADF augments the proliferation of HTLV-I and EBV transformed cells as an autocrine growth factor. These data are indicative of the possible involvement of ADF in virus-related transformation of cells and their autocrine growth. On the other hand, thioredoxin contains a redox active disulfide and has a reducing activity in the presence of thioredoxin reductase and NADPH. To clarify the role of ADF/thioredoxin system in the viral transformation, we tested the effect of 13-cis-retinoic acid (RA), which is a competitive inhibitor of thioredoxin reductase, on the growth of ADF high producing cells. The expression of IL-2R/alpha on HTLV-I (+) cells was suppressed by RA. RA dose-dependently reduced the cell number and viability of ADF high producing lymphoid cells. Moreover, it had a suppressive effect on the proliferation of ADF high producing cells. It is suggested that RA has an inhibitory effect on the activation and the growth of cells producing ADF and that inhibition of the ADF/thioredoxin system may be a new therapeutic approach for retrovirus-related disorders.