We compared the transcriptional activities of the circular and linear forms of short (4kbp) and giant (106kbp) DNA molecules in the presence of a polyamine, spermine (4+). With an increase in the spermine concentration, transcriptional activity was enhanced, followed by an inhibitory effect, and complete inhibition was observed in the sole case of long, linear templates. A difference between the transcriptional properties of circular and linear conformations is found for giant DNA molecules. These results are discussed in relation to DNA conformational transitions.