The expression of lck gene (lymphocyte specific protein tyrosine kinase) in the human system was examined by Northern blot analysis in human T cell leukemia virus type I (HTLV-I)-positive T cell lines, HTLV-I-T cell lines, normal T cell population, and a T cell line infected with human immunodeficiency virus. In the cells of HTLV-I-integrated T cell lines, messages of lck were not detected in IL-2-independent lines, although found profusely in IL-2-dependent ones. The results of nuclear transcription assay indicated that lck expression is shut off at the stage of transcription in those cell lines. RNA products of the viral PX region were not detectable in two out of five HTLV-I+, IL-2-independent lines, suggesting that PX gene products themselves exert no direct effect on the block of lck transcription in the HTLV-I-infected T cells. Other T cell populations and a T cell line infected with human immunodeficiency virus, on the other hand, showed significant levels of lck message. These data presented the possibility that lck gene product may be one of the intervening molecules which transduce the signal from the IL-2R into the cell interior, and play an important role in the pathophysiology of adult T cell leukemia, especially in the transition of these leukemias from the IL-2-dependent stage to IL-2-independent one for their growth.