maf is a family of genes encoding bZIP transcription factors. We isolated two cellular maf-related cDNAs, maf-1 (mafB) and maf-2 (c-maf), from rat and determined the specificities of DNA binding and heterodimer formation. Although both Mafs strongly bind to MARE, the consensus Maf recognition sequence (MARE, -TGCTGACTCAGCA-), originally identified by v-Maf protein Maf-1, recognizes a number of sequences containing only the first half of the MARE, -GCTGAC-. On the other hand, no such consensus short sequence could be determined for Maf-2. We determined the specificities of heterodimer formation with all members of the Jun and Fos family. In contrast to v-Maf which forms heterodimers with all Jun and Fos proteins, Maf-1 heterodimerizes with all four Fos proteins, but not at all with the three Jun proteins. Maf-2 heterodimerizes with c-Fos. We have also found that heterodimer formation of Maf-2 with c-Fos dramatically changes the specificity of DNA binding and trans-activation activity from that of the Maf-2 homodimer. These results show that Maf-1 and Maf-2 have significantly different properties and they might have different target genes and functions, in spite of the similarity of their bZip domain structure.