We have cloned a rat cDNA for a novel brain-derived immunoglobulin (Ig) superfamily molecule, BIG-1, by using PCR based on the amino acid sequences of the two closely related and well-known Ig superfamily members, rat TAG-1 and mouse F3. BIG-1 is a glycosylphosphatidylinositol-anchored membrane protein with six Ig-like domains and four fibronectin type III repeats, belonging to the TAG-1/F3 subgroup. The expression of BIG-1 mRNA is developmentally regulated with the highest level in the adult brain. It is restricted to subsets of neurons such as Purkinje cells of the cerebellum, granule cells of the dentate gyrus, and neurons in the superficial layers of the cerebral cortex. Recombinant BIG-1 protein has a neurite outgrowth-promoting activity when used as a substrate for neurons in vitro. These results suggest that BIG-1 may be involved in the formation and maintenance of neuron type-specific networks in the brain.