Two HTLV-I-carrying T cell lines were prepared from peripheral lymphocytes of a virus-infected (B/J X Chbb:HM) F1 rabbit, and these cells were inoculated intraperitoneally into 5 newborn F1 rabbits. These animals were killed 3-5 weeks later. Their leukocyte counts were higher than those in normal control animals, with abnormal lymphocytes amounting to 3-5% of total leukocytes. Histological examination showed leukemic infiltration in liver, spleen, lung and kidneys of all these animals. The peripheral lymphocytes at first lacked HTLV-I antigens, but became antigen-positive after in vitro culture. Southern blot analysis of these cells revealed HTLV-I integration patterns different from those of the inoculated cells.