We investigated tumor-associated antigens induced by infection with human T-lymphotropic virus type I(HTLV-I). Anaplastic large cell lymphoma (APLL) antigens were found to be expressed on interleukin 2 (IL-2)-dependent, HTLV-I-infected CD4+ T-cell lines established from patients with adult T-cell leukemia and HTLV-I-associated myelopathy/tropical spastic paraparesis. However, APLL antigens were not detected on unstimulated lymphocytes, mitogen-activated T lymphocytes, or Epstein-Barr virus-infected B cells. Furthermore, APLL antigens were not found on IL-2-independent HTLV-I-transformed cells such as MT-I or MT-2. When naive CD4+ T cells were infected with HTLV-I in vitro in the presence of IL-2, the APLL antigens were detected on them 4 weeks after infection. The expression level increased in a time-dependent fashion. These results indicate that HTLV-I infection induces a unique category of tumor-associated antigens on CD4+ T cells.