B-cell lines were established as spontaneous outgrowths of cell cultures from patients with adult T-cell leukemia (ATL). Three such lines were shown to have integrated human T-cell leukemia/lymphoma virus 1 (HTLV-I) proviral sequences as well as Epstein-Barr virus (EBV) infection. Supernatant fluids from these cultured cells were assayed for interferon (IFN) production. Acid-stable alpha-IFN was found to be produced by one cell line (CF), and acid-labile alpha-IFN by the other two (HS, MJB). In contrast, HTLV-I-infected T-cell lines did not produce IFN. Some EBV-infected B-cell lines produce acid-labile alpha-IFN, while others do not. Since alpha-IFN, both acid stable and acid labile, is found in sera from patients with the polyclonal activation of B cells as a constitutive part of the disease, the above observations suggest a possible role of polyclonal B-cell activation in alpha-IFN production in these diseases.