By virtue of the protein's size, myofibrillar localization, and proposed functional role, the gene encoding the giant sarcomere matrix protein nebulin represents a possible site for myopathic mutations. Using polyclonal anti-nebulin antisera to screen a cDNA expression library, we have isolated and characterized two separate human fetal muscle cDNA clones. By recovering fusion polypeptide-bound portions of our polyclonal antiserum and reutilizing them to probe Western blots, we further demonstrate that the expressed cDNAs encode polypeptide epitopes unique to the protein nebulin. Both cDNAs detect a 25-kb skeletal muscle RNA transcript and localize to human chromosome 2. The identification of nebulin cDNA clones enables the complete analysis of this enormous mRNA by transcript walking through muscle cDNA libraries. Here we report a restriction map of the 3' end of the human nebulin transcript, with reference to the genomic fragments identified by the cDNA.