Adult T-cell leukemia (ATL) cells have been shown to express the receptor for IL-2 by studies using anti-CD25 monoclonal antibody, but these cells usually show no or only a weak proliferative response to IL-2. In the present study, we established thirteen IL-2-dependent T-cell lines from four ATL patients. Examination of the clonalities of these cell lines by the rearrangement profiles of the TCR beta-chain gene and the integration sites of the HTLV-I proviral genome, revealed that two cell lines (KK-1 and KK-5) were of real ATL cell origin. The others were of normal T-cell origin and had been established by infection with HTLV-I. The KK-1 and KK-5 cell lines were derived from a single ATL patient (KK). Interestingly, these cells showed different phenotypic features from the majority of original leukemia cells (CD3 +/- CD4+ CD8-). The KK-1 cell line acquired CD8 antigen expression and became double-positive (CD3 +/- CD4+ CD8+), while the KK-5 cell line prominently expressed CD3 antigen (CD3+ CD4+ CD8-). These results indicate that the phenotypic feature of ATL cells are not fixed, but can change in vitro as has occasionally been observed in vivo.