The antibiotic distamycin A is a DNA minor groove binding drug (MGB) that recognizes a stretch of at least four ATs. The alkylating benzoyl mustard derivative tallimustine (FCE 24517) has powerful anti-tumor activity. Using the electrophoretic mobility shift assay (EMSA) we determined that both compounds can prevent binding of TBP and, with 10-fold higher concentration, TBP-TFIIA (DA) and TBP-TFIIA-TFIIB (DAB) to a TATA box. Once formed, the DA and DAB complexes are more resistant to MGB challenge. Both drugs can inhibit basal in vitro transcription of a minimal TATA-containing promoter and similar concentrations are necessary for binding and transcriptional inhibition. Tallimustine shows strong selectivity by decreasing only correctly initiated transcripts. Even at high doses (20 microM), however, they cannot disturb a competent pre-initiation complex or Pol II progression. This functional in vitro model will provide a way to investigate the activity of sequence-specific DNA binding drugs with potential anti-viral and anti-tumour activity and to develop novel more selective compounds.