An autocrine mechanism of proliferation may play a significant role in the leukemogenesis of adult T-cell leukemia, a mature T-cell malignancy caused by human T-lymphotropic virus type I (HTLV-I). To further delineate the role of HTLV-I and the interleukin 2 (IL2) system in the transformation process, human primary lymphocytes were infected by cocultivation with lethally X-irradiated MT2 cells in the presence or absence of human rIL2; the polymerase chain amplification reaction was used to examine quantitatively the expression of HTLV-I, IL2, and IL2R alpha mRNAs during early and late proliferation phases that displayed polyclonal (days 7 to 49) and oligoclonal (days 100 to 150) proviral integration, respectively. IL2 mRNA and IL2 activity were transiently expressed during the polyclonal phase but were undetectable at later time points. IL2R alpha mRNA expression remained at a constitutively high level throughout the examined time course. Viral transcripts were detectable at each time point. Expression of the tax-rex mRNA was inversely related to IL2 mRNA levels; it was low early in the polyclonal phase but increased 30-fold with the development of oligoclonality. In addition, paraformaldehyde-fixed HTLV-I-producing cells activated peripheral blood lymphocytes. These data suggest that HTLV-I activates human T lymphocytes. However, IL2 expression is transient, indicating a limited involvement of an IL2 autocrine growth loop in the transformation process. Lastly, another viral determinant, in addition to the trans activator tax, may be important in HTLV-I-induced T-cell proliferation.