c-Myc protein contains a basic helix-loop-helix and leucine zipper (bHLHLZ) structure in its carboxyl terminal region, forms heterodimers with Max, and binds to the CACGTG sequence in DNA. A number of bHLHLZ proteins are present in cells, and some of them bind to the same DNA sequence. Using an anti c-Myc monoclonal antibody, MYC5, raised against a bacterially synthesized c-Myc protein, we have carried out immunoaffinity purification of c-Myc proteins from cultured mammalian cells. The immunoaffinity-purified fraction was found to contain not only c-Myc but also other CACGTG sequence-binding proteins, such as Max, Mad, and USF, indicating a wide cross-reaction to CACGTG sequence-binding proteins. The MYC5 antibody recognized the common structural motif in their basic region required for sequence-specific DNA binding and was shown to inhibit their DNA-binding activities. The immunoaffinity-purified N-Myc, Max, Mad, and, presumably, c-Myc were highly phosphorylated, and phosphatase treatment increased the DNA-binding activity of Myc, suggesting that the DNA-binding activity of c-Myc was regulated by phosphorylation in vivo. From these results, we can conclude that the MYC5 antibody constitutes a powerful tool for the purification and characterization of c-Myc and other CACGTG sequence-binding proteins.