Tax protein activates transcription of the human T-cell leukaemia virus type I (HTLV-I) genome through three imperfect cyclic AMP-responsive element (CRE) target sites located within the viral promoter(1). Previous work has shown that Tax interacts with the bZIP element of proteins that bind the CRE target site(2-4) to promote peptide dimerization(3,5), suggesting an association between Tax and the bZIP coiled coil. Here we show that the site of interaction with Tax is not the coiled coil, but the basic segment. This interaction increases tbe stability of the GCN4 bZIP dimer by 1.7 kcal mol(-1) and tbe DNA affinity of the dimer by 1.9 kcal mol(-1). The differential effect of Tax on several bZIP-DNA complexes that differ in peptide sequence or DNA conformation suggests a model for Tax action based on stabilization of a distinct DNA-bound protein structure. This model may explain how Tax interacts with transcription factors of considerable sequence diversity to alter patterns of gene expression.