Effect of daunomycin on the higher order structure of single giant DNA molecule compacted by spermidine was studied using fluorescence and electron microscopies. We found that, with the addition of daunomycin, unfolding of the compacted DNA is induced, indicating that this drug has the activity to loose the higher order structure of the compacted state. Such an effect will be concerned with the antitumor activity of daunomycin.