The bHLHZip protein Max interacts with both the Myc and Mad family proteins forming heterodimers which specifically bind certain E-box DNA recognition sequences, thereby regulating transcription. Whereas Myc proteins actively promote cell proliferation, Mad complexes have the opposite function. Although the main regulation of this network seems to be the control of myc- and mad family gene expression, regulation at the level of DNA-binding and transactivation may also be in operation. Few studies on the DNA-binding activity of native Myc:Max or Max:Mad complexes have been reported mainly due to technical difficulties. To overcome these problems we have developed a specific and sensitive solid phase DNA-binding assay based on partial purification of native Myc, Max and Mad1 complexes by immunological methods. Using this technique we report that the DNA-binding activity of c-Myc-containing complexes is reduced during induced differentiation of U-937 monoblasts and F9 embryonic teratocarcinoma cells. In contrast, the DNA-binding of Mad1-containing complexes increases during monocytic differentiation. In general, the DNA-binding activity of c-Myc and Mad1 correlate with their expression. However, our studies of early kinetics of TPA-induced differentiation of U-937 cells as well as of late events during F9 differentiation suggest that post-translational regulation of Myc and Max DNA-binding may also occur. The solid phase DNA-binding assay may thus provide a tool to study the regulation of DNA-binding in more detail.