Rabbit antibodies raised against a 135- to 140-kD glycoprotein isolated from the culture medium of mouse forebrain explants were used for the identification and cloning of a complex of mouse neural cell surface glycoproteins. The antibodies recognized a 135-kD surface protein which shared the L2/HNK-1 epitope with several neural cell adhesion molecules. Three homologous complementary deoxyribonucleic acid (cDNA) clones were isolated from a mouse brain cDNA library prepared in the expression vector lambda gt11, one of which was sequenced and found to lack sequence homologies with known proteins. In Northern blots, this clone hybridized with a single 6.3 kb messenger ribonucleic acid (mRNA). In immunoblots of mouse brain extracts, antibodies raised in rabbits against the fusion protein encoded by it stained two glycoproteins of 135 and 90 kD, which we designated F3.135 and F3.90. In the developing mouse cerebellum, F3 antigenic sites were found predominantly on parallel fibers and on postmitotic neurons. In fetal brain cell cultures, F3 antigen was detected at the surface of cells with neuronal morphology, but the antibodies also stained some non-neuronal cells in a pattern characteristic of matrix components. Because all proteins carrying the L2/HNK-1 epitope identified so far have a role in cell adhesion, it can be anticipated that the F3 surface proteins also are involved in cell-interaction phenomena.