The c-Myc oncoprotein (Myc) functions as a transcription regulator in association with an obligatory partner, Max, to control cell growth and differentiation. The Myc:Max complex regulates specific genes by recognizing "E-box" DNA sequences and promoter-bound factors such as Miz-1. Myc recruits histone acetyltransferases (HATs) to modify chromatin and is, itself, acetylated in mammalian cells by several of these HATs including p300/CBP, GCN5, and Tip60. The Myc residues that are directly modified by these different HATs remain unknown. Here, we have analyzed the acetylation of recombinant Myc:Max complexes by purified p300 HAT in vitro by using MALDI-TOF and LC-ESI-MS/MS mass spectrometry. These analyses identify six lysine residues in human Myc (K143, K157, K275, K317, K323, and K371) as direct substrates for p300. Our results further indicate that p300 can acetylate DNA-bound Myc:Max complexes and that acetylated Myc:Max heterodimers efficiently interact with Miz-1.