The complete amino acid sequence of muscle desmin reported here is the first for an intermediate filament protein. Alignment with partial data available for vimentin, glial fibrillary acid protein, neurofilament 68 K, two wool alpha-keratins, and a recently described DNA clone covering 90% of an epidermal keratin shows that all seven proteins have extensive homologies and therefore form a complex multigene family, the intermediate filament proteins. The hard alpha-keratins of wool appear to be a special subset of epithelial keratins. The sequence information reveals, as the dominant structural principle, a rod-like middle domain arising from several alpha-helical segments able to form interchain coiled-coil elements. The proposed helices are separated by short spacers, which like the two terminal domains seem built from non-alpha-helical material. Attention is drawn to the sometimes very striking sequence homologies along the rod and the high sequence variability in the terminal domains. Finally, chemical cross-linking experiments performed on the isolated desmin rod show that intermediate filament structure seems not to be based on triple-stranded coiled-coils as currently thought, but rather reflects protofilament units built as a dimer of normal interchain double-stranded coiled-coils.