H uman T lymphotropic virus type I (HTLV-I) is an exogenous RNA tumor virus etiologically linked to adult T cell leukemia and related diseases. In this paper, we describe that two 2',3'-dideoxynucleoside analogues, erythro 3'-azido-2',3'-dideoxythymidine (also called azidothymidine) and 2',3'-dideoxycytidine can inhibit the infectivity of HTLV-I against helper/inducer T cells in vitro. Both 2',3'-dideoxynucleoside analogues inhibited the overgrowth of target T cells, which was a consequence of virally mediated transformation, when they were exposed to the virus and cultured with the compounds. A profound decrease in the expression of HTLV-I gag-proteins was also observed. Moreover, we observed that the amount of proviral DNA detected in cellular DNA from the target T cells was substantially reduced when the cells were protected by the compounds against the virus and that at certain concentrations of the compounds the synthesis of viral DNA was completely suppressed. These results may be of value in developing a new pharmacological strategy for preventing the replication and possibly blocking the transmission of HTLV-I and related retroviruses in human beings.