Max is a small helix-loop-helix protein which forms heterodimers with members of the Myc protein family. Myc/Max heterodimers exhibit sequence-specific DNA binding with much greater affinity than Myc homodimers. The Xenopus laevis homologue of Max, XMax, is shorter than the equivalent mammalian protein. This difference results from the deletion of a 24 amino acid sequence located near the C-terminus of the protein. Xenopus max transcripts undergo alternative splicing. In addition to the 27 base alternatively spliced sequence (exon A) previously detected in mice and humans, some Xmax transcripts also contain an 81 base sequence (exon B) at a second site within the coding sequence. Although exon B insertion alters part of the leucine zipper protein/protein interaction domain, the resulting XMax protein retains the ability to form stable heterodimers with both c-Myc and N-Myc. Xmax mRNA is present at approximately constant levels during early development. This contrasts with the rapidly changing levels of c-myc and N-myc mRNA in the embryo and has implications for regulation of gene expression during differentiation. All four alternatively spliced forms of Xmax mRNA are present during development and in all adult tissue examined.