To understand how the growth of T-cells transformed by Human T-cell leukemia virus type I (HTLV-I) is deregulated, we analysed the expression of cell-cycle regulatory genes in HTLV-I infected and non-infected T-cell lines. We investigated the gene for 6 cyclins, 4 cyclin-dependent kinases, and 5 cyclin-dependent kinase inhibitors, and found the following:(1) HTLV-I infected T-cell lines preferentially expressed cyclin D2, whereas cyclin D3 was the major D-type cyclin in HTLV-I negative T-cell lines;(2) HTLV-I infected T-cell lines expressed strikingly low levels of p18Ink4 compared with those that were HTLV-I negative;(3) HTLV-I infected T-cell lines expressed high levels of p21Waf1/Cip1/Sdi1, whereas p21Waf1/Cip1/Sdi1 was undetectable in HTLV-I negative T-cell lines. These features were also found in T-cells immortalized by Tax1, which we established. Therefore, it is strongly suggested that Tax1 alters the expression of these cell-cycle regulatory genes.