The condensation of a single DNA can be induced by multivalent counterions and is thought to be due to ion-ion correlations. Both Monte Carlo simulations and fluorescence microscopy experiments confirm this picture. The effect is less pronounced for chain counterions (polyamines) than for simple spherical counterions with the same charge. When simple salt is added to the system, the polyelectrolyte unfolds. MC simulations show that this is due to a competition between the different small ion species. The multivalent ions in the vicinity of the macroion are exchanged for univalent ones, decreasing the effect of ion-ion correlations. It is also found that the temperature effect on the macroion conformation is quite insignificant in the studied interval(5.0-65.0 degrees C).