Polyamine ions such as spermidine 3+ , along with monovalent and divalent salt ions, screen the negatively charged backbone of dsDNA and thereby facilitate processes in which DNA is confined in small spaces, such as viral DNA packaging. We use optical tweezers to directly manipulate single DNA molecules and have made preliminary measurements of the effect of spermidine on DNA elasticity, condensation, and viral packaging. We determine the concentration of spermidine 3+ at which dsDNA condenses in the presence of Mg 2+ and Na + and report a monotonic increase in stretch modulus and decrease in persistence length at incremental spermidine concentrations up to the concentration at which dsDNA condenses. We also discuss the effect of spermidine on DNA packaging in bacteriophage phi29.