Several monoclonal antibodies (mAb) reactive against a high-molecular- weight growth factor from human glioblastoma cell lines have been produced by immunizing mice with partially purified preparations from conditioned media. Antibody-secreting colonies were selected by their capacity to bind 35S- labeled glioma cell protein and by reactivity in indirect enzyme-linked immunoadsorbent assay (ELISA), using high-molecular-weight gel filtration fractions and preparative isoelectric focusing fractions containing growth factor activities. Two of the select mAbs (20F3 and 12A12) depleted mitogenic activity (>50% inhibition, p < 0.05) from gel filtration fractions by immunoprecipitation, but could not neutralize mitogenic activity directly. Mitogenic activity recovered from affinity columns prepared with mAb 20F3 eluted at 48% and 52% acetonitrile from HPLC C4 reversed-phase columns. Immunoprecipitation of 35S-labeled cell lysates with 20F3 followed by resolution with SDS-PAGE autoradiography revealed one unique protein of 170 kD. Established glioma cell line D-54 MG showed perinuclear and cytoplasmic staining with mAb 20F3. mAb 20F3 should prove useful in purification and characterization of these glioma-derived growth factor(s).