It has previously been determined that transformed human B cells may be infected by and retain integrated human T cell lymphotropic virus, type I (HTLV-I). Although HTLV-I is primarily associated with transformation of human T lymphocytes, immortalized B cell populations have arisen after cocultivation of normal B cells and irradiated HTLV-I-infected T cells. To test whether HTLV-I infection might be involved in this B cell transformation process, we characterized five independent HTLV-I-infected and -immortalized human B cell clones. All five clones contained clonal HTLV-I integrations, expressed cell surface IgM, and secreted IgM in quantities varying from 0.1 to 4.0 micrograms/ml. Immunoblotting and immunoprecipitation of metabolically labeled cell lysates failed to detect synthesis of HTLV-I gag, env, or tat gene products, and cellular RNA dot blot analysis detected varying levels of HTLV-I gene transcripts in only three of the five clones. The secreted IgM from culture supernatants were affinity purified, and were found to selectively immunoprecipitate an acidic protein (isoelectric point = 5.0) of 66,000 m.w. (p66) from purified radioiodinated HTLV-I virions. This p66 copurified with metabolically labelled HTLV-I-infected B cell IgM from HKA-3 cells. Although HTLV-I RNA transcripts were present at low levels, the absence of HTLV-I proteins in HKA-3 cells made it unlikely that p66 was related to the major HTLV-I envelope glycoprotein, gp62. Anti-idiotypic mAb directed against the IgM produced by one B cell clone (HKA-3), as well as purified HTLV-I virions (containing p66), stimulated HKA-3 cell proliferation. Preincubating the anti-Id antibody or HTLV-I with excess HKA-3 IgM abolished the binding of either to HKA-3 cells. These data suggest that HTLV-I infected cells produce a cellular protein (p66), which is incorporated into and copurifies with HTLV-I virions, and which in at least one case (HKA-3) may act as a mitogenic stimulus, potentially contributing to the HTLV-I mediated transformation process.