Fine structural and immunocytochemical characterization of rabbit lymphoid cell lines transformed by human T-lymphotropic virus type I (HTLV-I) was carried out. All nine cell lines tested were reactive with anti-HTLV-I-positive human, monkey, and rabbit sera and monoclonal antibody to HTLV-Ip 19, but not with anti-HTLV-I-negative sera and monoclonal antibodies to human Ia and pan-T antigens. All cell lines were strongly positive for monoclonal antibodies to rabbit Ia and pan-T antigens. Ultrastructurally, each cell line contained C-type virus particles in varying numbers in the extracellular space. These particles showed replication patterns similar to those in HTLV-I or simian T-lymphotropic virus type I (STLV-I)-producing human or monkey cells. In addition, anti-HTLV-I-positive rabbit serum gave positive immunoreactivity to HTLV-I or STLV-I by indirect immunoferritin method. These results indicate that the ultramorphology and replication patterns of HTLV-I in rabbit cell lines are indistinguishable from those of HTLV-I in human and monkey cell lines, HTLV-I in rabbit cells shares the common surface antigenic determinants with HTLV-I or STLV-I in human or monkey cells, and that these cells are definitely rabbit T cells bearing their own Ia antigens.