The effect of acetylation on the DNA binding properties of the rigidly conserved histone H4 amino-terminal tail has been studied in detail using the technique of thermal denaturation. The quantitative DNA-binding parameters for both the non- and fully acetylated H4 amino terminus have been determined from thermal denaturation data for complexes of the peptides bound to mixed sequence 146-base pair DNA. We find that under dilute buffer conditions (5 mM Tris-HCl) the binding constant for the non-acetylated peptide to double-stranded DNA is 5 x 10(11) M-1 and that acetylation of lysine residues in the peptide reduces the binding constant to 1 x 10(5) M-1. The dramatic differences observed in the binding constants for the non- and fully acetylated peptides are probably due to the effect of acetylation on the even distribution of positively charged residues in the H4 amino terminus. In other experiments, the binding of both peptides to a 30-base pair oligonucleotide has been studied in solution with varying concentrations of sodium, magnesium, and phosphate ions. These experiments demonstrate that both magnesium and phosphate ions have strong effects on the binding of the H4 tail to DNA, especially weakening the binding of the acetylated peptide. For instance, the dissociation of the non-acetylated peptide from DNA requires 6 mM magnesium, yet the binding of the acetylated peptide is abolished in only 30 microM magnesium. The modulation of the DNA binding interactions of the H4 amino terminus by physiologically relevant ionic conditions, in addition to the effect of acetylation, can be important in the regulation of chromatin structure and function.