Monoclonal antibody (mAb) 5B4 recognizes a developmentally regulated membrane glycoprotein (Mr approximately 185,000-255,000) expressed on sprouting neurons. The amino acid sequence deduced from lambda gt11 cDNA clones encoding the transmembrane and cytoplasmic domains of the 5B4 antigen is co-linear with that of chick NCAM-ld. The significant level of overall sequence identity (75%) demonstrates that the 5B4 antigen is rat brain NCAM. The 5B4 epitope maps to the carboxy-terminus common to both NCAM-ld and NCAM-sd.