Using a BAL31 exonuclease assay to determine the sites of 4,5',8-trimethylpsoralen photocrosslinking in DNA we have shown that 5'-TA sites which are accessible to psoralen DNA interstrand photocrosslinking in naked DNA become inaccessible when protein, in casu, lambda-repressor E. coli or RNA polymerase are bound at their recognition DNA sequences (OR1 operator or deo1 promoter, respectively). These results show that psoralens can be used as photofootprinting reagents to study specific protein-DNA interactions.