A neuronal heparan sulfate proteoglycan was identified by a panel of four monoclonal antibodies. The antibodies were generated from mice immunized with embryonic chick retina basal lamina (clones 3A12, 3A3, and 9E10) and embryonic chick optic tract (clone 6D2). Cross-reactivity of all four antibodies with the purified proteoglycan confirmed that the antibodies were directed to the same antigen. Antibodies to heparan sulfate proteoglycan from embryonic chick muscle or EHS mouse tumor (perlecan) did not cross-react with the neuronal heparan sulfate proteoglycan, suggesting that the two proteoglycans are not related. In Western blots, the proteoglycan had a molecular weight of 600 kDa that dropped to 250 kDa when the samples were treated with heparitinase or nitric acid. Immunocytochemistry showed that in early stages of chick and quail development, the proteoglycan was exclusively localized in basal laminae and had a distribution similar to that of laminin. During further development, a strong labeling was also found in the extracellular environment of nerve tracts, such as the optic nerve and white matter areas of the brain and spinal cord. The labeling of axonal tracts declined from embryonic day 10 onward, while labeling in basal laminae persisted. Antibodies to muscle heparan sulfate proteoglycan or to perlecan did not label nerve fibers. The data show that embryonic neuronal tissue expresses a new type of heparan sulfate proteoglycan.