Neural adhesion molecules in the immunoglobulin superfamily play essential roles in axonal guidance during development, plasticity, and maintenance of synaptic connections in the adult brain. Recently, we reported two novel cDNAs encoding adhesion molecules, NB-2 and NB-3, in the contactin/F3 subgroup of the immunoglobulin superfamily from rat brain. We have now isolated cDNA encoding human NB-3. The cDNA clone, hNB-3, consists of 3,530 nucleotides with an open reading frame of 3,084 nucleotides encoding 1,028 amino acids. It shares with rat NB-3 86% identity in nucleotide sequences and 90% identity in amino acid sequences. Likewise, hNB-3 exhibits 53% and 51% identity in nucleotide sequences and 43% and 44% identity in amino acid sequences with human contactin/F3 and human TAG-1/axonin-1, respectively. Northern blot analysis of mRNA isolated from different regions of the adult human nervous system showed that the hNB-3 mRNA content was regionally different by dozens-fold, although the mRNA was detected in all regions, as a transcript of 3.7 kb. The cerebellum showed the highest expression of hNB-3 mRNA among various regions of the nervous system. Chromosomal localization of hNB-3, using fluorescence in situ hybridization, was assigned to 3p25-26.