The behavior of alkaline earth metal cations (Mg2+ and Ca2+) and transition metal cations (Zn2+ and Cu2+) interacting with lambda-DNA-HindIII fragments ranging from 2,027 to 23,130 bp in Tris-borate-EDTA buffer solutions was investigated. The divalent counterions competed with Tris+ and Na+ for binding to polyion DNA, and the competition binding situations were investigated by measuring the reduction of the DNA mobility, by pulsed- or constant-field gel electrophoresis. The interaction of Mg2+ with DNA was intensively studied over a wide range of Mg2+ concentrations. In addition, we examined the competition binding as a function of ionic strength and DNA size. To compare valence effects, we studied Co(NH3)6(3+) interaction with DNA fragments under conditions similar to that of Mg2+. At relatively low Mg2+ concentration, the normalized titration curves of DNA mobility were well fit by Manning's two-variable counterion condensation (CC) theory. The agreement between the predicted value (total charge neutralization fraction theta) from Manning's CC theory and the data based on our measured DNA electrophoretic mobility reduction was consistent under our experimental conditions. In contrast to alkaline earth metal cations (Mg2+ and Ca2+), different binding behaviors were observed for the transition metal cations (Zn2+ and Cu2+). These differences highlight the usefulness of our reduced DNA electrophoretic mobility measurement approach to describing cation interactions with polyelectrolyte DNA.