We describe an assay for detecting intramolecular triple-stranded DNA in living cells. The assay involves quantitative analysis of the differential photobinding of 4,5',8-trimethylpsoralen to 5'TA and 5'AT dinucleotides in a region of plasmid DNA containing the triplex-forming sequence (GA)7TA(GA)7. Psoralen photobinds to the central 5'TA within the (GA)7TA(GA)7 sequence in duplex DNA but not when the sequence exists as an intramolecular triplex structure. The reactivity of photobinding sites in regions flanking the intramolecular triplex-forming sequence, those that comprise the duplex-triplex junctions, either increase or decrease upon formation of the intramolecular triplex structure from duplex DNA. The pattern of trimethylpsoralen reactivity provides an indication of the conformation of the (GA)7TA(GA)7 sequence in plasmid DNA. The formation of the intramolecular triplex structure is dependent on both pH and negative superhelical density in vitro. The fraction of the (GA)7TA(GA)7 sequence that existed as an intramolecular triplex structure in vivo was dependent on the level of DNA supercoiling in vivo and changes in growth conditions which influence the intracellular pH. The Hy3 isomer was detected in living Escherichia coli cells.