A key problem in nervous system development is how distinct subpopulations of progenitor cells give rise to different adult brain structures. The labeling pattern of the FORSE-1 antibody subdivides the neuroepithelium of the embryonic forebrain into domains resembling those of certain transcription factors, suggesting that the FORSE-1 epitope may be involved in the specification of development compartments. Therefore, it is important to determine the identity of the antigen(s) recognized by FORSE-1. On immunoblots, FORSE-1 recognizes a single, high-molecular-weight species, which we have identified as phosphacan, a brain-specific chondroitin sulfate proteoglycan that binds neural cell adhesion molecules. This identification is based on cross-immunoprecipitations and immunoblotting using an anti-phosphacan antibody and FORSE-1. FORSE-1 also recognizes two major neutral glycolipids in embryonic brain. The FORSE-1 epitope is sensitive to endo-beta-galactosidase, suggesting that the epitope corresponds to a carbohydrate moiety. Moreover, immunoprecipitates of the proteoglycan bearing the FORSE-1 epitope bind antibodies that recognize the Le* carbohydrate, and immunostaining patterns of embryonic brain sections by FORSE-1 and a known anti-Le* antibody are identical. Finally, purified FORSE-1 specifically recognizes Le*-containing glycoconjugates in ELISAs. The pattern of FORSE-1 labeling, the identification of its epitope as Le*, which has implicated in cell adhesion, and the presence of Le* on phosphacan suggest that this carbohydrate epitope may play a role in adhesive interactions important for proliferation, cell migration, or axon guidance.