The results presented describe the effects of various spectator ligands, attached to a platinum 1,2-intrastand d(GpG) cross-link in duplex DNA, on the binding of high-mobility-group box domains (HMGB domains) and the TATA-binding protein (TBP). In addition to cisplatin-modified DNA, 15-base pair DNA probes modified by {Pt(1R,2R-diaminocyclohexane)}²⁺, cis-{Pt(NH[sub3})(cyclohexylamine)}²⁺, {Pt(ethylene-diamine)}²⁺, cis-{Pt(NH₃)(cyclobutylamine)}²⁺, and cis-{Pt(NH₃)(2-picoline)}²⁺ were examined. Electro-phoretic mobility shift assays (EMSAs) show that both the A and B domains of HMGB1 as well as TBP discriminate between different platinum-DNA adducts. HMGB1 domain A is the most sensitive to the nature of the spectator ligands on platinum. The effect of the spectator ligands on protein binding also depends highly on the base pairs flanking the platinated d(GpG) site. Double-stranded oligonucleotides containing the AG*G*C sequence, where the asterisks denote the sites of platination, with different spectator ligands are only moderately discriminated by the HMG box proteins and TBP, but the recognition of dsTG*G*A is highly dependent on the ligands. The effects of HMGB1 overexpression in a BG-1 ovarian cancer cell line, induced by steroid hormones, on the sensitivity of cells treated with [Pt(1R,2R-diaminocyclohexane)Cl₂] and cis-[Pt(NH₃)(cyclohexylamine)Cl₂] were also examined. The results suggest that HMGB1 protein levels influence the cellular processing of cis-{Pt(NH₃)(cyclohexylamine)}²⁺-, but not {Pt((1R,2R)-diaminocyclohexane)}²⁺-, DNA lesions. This result is consistent with the ob-served binding of HMGB1a to platinum-modified dsTG*G*A probes, but not with the binding affinity of HMGB1a and HMGB1 to platinum-damaged dsAG*G*C oligonu-cleotides. These experiments reinforce the importance of sequence context in platinum-DNA lesion recognition by cellular proteins.