Newborn WKA rats given a single intraperitoneal injection of MMC treated TARS-1, a rat T cell line producing HTLV-I, were shown to accumulate high titred antibodies specific for HTLV-I. Adult WKA rats rejected TARS-1 transplant with transient appearance of anti-HTLV-I antibodies. However, rats maintained under daily administration of Bredinin, an immunosuppressive drug after TARS-1 transplant showed continuous production of antibodies specific for HTLV-I by aging. Type-C virus particles similar to HTLV-I were demonstrated by electronmicroscopy in the short-term cultured spleen cells of these rats. The evidence indicates that HTLV-I can be transmitted into newborn and immunosuppressed adult rats and they may provide a suitable animal model of ATL and related conditions in man, especially for elucidating the virus-host interactions involved in the leukemogenesis of HTLV-I. By using monoclonal antibodies, cell surface antigens associated with HTLV-I were also analysed.