A glioma-associated antigen was previously identified on an avian sarcoma virus-induced F-344 rat astrocytoma cell line S69-c15 by four rat monoclonal antibodies (7G4, 9F1, 10E3 and 10E7) produced after syngeneic immunization. Earlier data suggested all four antibodies reacted with a polypeptide-associated epitope. We report here that the antigen activity was detected in the supernatant of tumor homogenates and could pass through a 1000 Da molecular weight cut-off dialysis membrane, as determined by antibody binding inhibition in a cell surface radioimmunoassay. When the dialysate was fractionated by Bio-Gel P-2 chromatography, antibody inhibiting activity eluted in the range of 300-600 Da. A highly purified material was further isolated by ion exchange high pressure liquid chromatography. Parallel purification product from an antigen-negative cell line failed to demonstrate antibody inhibiting activity. We conclude that greater than 400-fold purification enrichment of antigen can be achieved. We postulate that the partially purified antigenic determinant is a glioma-associated determinant of highly restricted expression and is presented in hapten-carrier form by the glioma cells.