Rabbits were successfully infected with human T-cell leukemia virus type I (HTLV-I) and produced antibodies to adult T-cell leukemia-associated antigens (ATLA) on intravenous inoculation of the HTLV-producing human cord T-cell line MT-2, or autologous cell lines established by cocultivation with MT-2 cells. Lymphocytes taken from the rabbits between 4 and 14 days after the inoculation of MT-2 cells, but not lymphocytes obtained in earlier or later periods, could be immortalized in vitro and expressed ATLA and type C virus particles. However, lymphocytes harvested during an early culture period (5 to 8 days) were found to be negative for ATLA. The transformed cells had the karyotype of a normal male rabbit. Two rabbits inoculated with the autologous HTLV-producing cell lines did not allow their growth in vivo, but some peripheral blood lymphocytes could be immortalized in vitro. One of these transformed cell lines was examined for the integration of HTLV-I provirus genome. The transformed cells were found to contain the provirus genome and also to be monoclonal with respect to the integration site of the provirus genome, unlike the inoculated cells.