Single molecules of DNA can be visualized in solution by epifluorescence microscopy, manipulated and extended by a variety of mechanical, electrical and chemical means as described previously. This has been used to design experiments under an optical microscope, in which DNA molecules are extended by a known force, to determine the elastic response of the molecules, both in the presence and absence of ethidium bromide. It is found that at lower forces (smaller extensions) the molecules behave as entropic springs with a persistence length of 500 angstroms, and that at the ionic strengths used, the intercalation of ethidium bromide does not alter this persistence length, while it appears to elongate the contour length of the molecule by about 30%.