Human tumor suppressor protein p53 is a 393-amino acid phosphoprotein that enhances transcription in response to DNA damage from several genes that regulate cell cycle progression. The tetrameric state of p53 is critical to wild-type function; the p53 tetramerization element is located in the C-terminal region of the protein. This region is phosphorylated at several evolutionarily conserved serines, suggesting that phosphorylation may be an important regulator of p53 function. In order to determine the effect of phosphorylation on tetramer formation, we synthesized phosphopeptides corresponding to p53(Ser303-Asp393) with phosphate incorporated at Ser315, Ser378, or Ser392, and at both Ser315 and Ser392. Equilibrium ultracentrifugation analysis showed that phosphorylation at Ser392 increased the association constant for tetramer formation nearly ten-fold. By itself, phosphorylation at Ser315 or Ser378 had little effect on tetramer formation, but Ser315 largely reversed the effect of phosphorylation at Ser392. Analysis by calorimetry suggests that phosphorylation may influence subunit affinity by an enthalpy driven process.