Previously, we showed that surface expression of intercellular adhesion molecule 1 (ICAM-1) was strongly upregulated in T cells carrying proviral human T-cell leukemia virus type 1 (HTLV-1) and that the viral transactivator protein Tax1 was capable of inducing the ICAM-1 gene. To determine the responsive elements in the human ICAM-1 gene promoter, a reporter construct in which the 5'-flanking 4.4-kb region of the ICAM-1 gene was linked to the promoterless chloramphenicol acetyltransferase (CAT) gene was cotransfected with expression vectors for Tax1 and Tax2, both of which were separately confirmed to be potent transactivators of the HTLV-1 long terminal repeat (LTR). Tax1 strongly activated the ICAM-1 promoter in all the cell lines tested: three T-cell lines (Jurkat, MOLT-4, and CEM), one monocytoid cell line (U937), and HeLa. Unexpectedly, Tax2 activated the ICAM-1 promoter only in HeLa. By deletion and mutation analyses of the 1.3-kb 5'-flanking region, we found that Tax1 transactivated the ICAM-1 promoter mainly via a cyclic AMP-responsive element (CRE)-like site at -630 to -624 in the Jurkat T-cell line and via an NF-kappaB site at -185 to -177 and an SP-1 site at -59 to -54 in HeLa. On the other hand, Tax2 was totally inactive on the ICAM-1 promoter in Jurkat but transactivated the promoter via the NF-kappaB site at -185 to -177 in HeLa. Gel mobility shift assays demonstrated proteins specifically binding to the CRE-like site at -630 to -624 in Tax1-expressing T-cell lines. Stable expression of Tax1 but not Tax2 in Jurkat subclones enhanced the surface expression of ICAM-1. The differential ability of Tax1 and Tax2 in transactivation of the ICAM-1 gene may be related to the differential pathogenicity of HTLV-1 and HTLV-2.