The present study describes identification and partial characterization of a glioma-derived high molecular weight transforming growth factor beta-like molecule (HMW-TGFβ that requires no activation for biological activity. HMW-TGFβ constitutively produced by the human glioma cell line, D54MG, is not acid- or heat-labile; is relatively resistant to denaturation, reduction, and high salt treatment. Monoclonal antibody 12A12.D7, produced against partially-purified HMW-TGFβ was used both to deplete and to neutralize directly a > 158 kDa HMW-TGFβ activity from gel filtration fractions; the antibody also directly neutralized purified mature TGFβ1. 12A12.D7 recognized a single protein species of 186 kDa from unlabeled glioma cell conditioned media and 35S-labeled lysates. HMW-TGFβ is not due to complex formation between TGFβ and any of the known carrier molecules. Production of HMW-TGFβ by glioma cells could facilitate tumor cell proliferation, and thus contribute to the inexorable and rapid progression that characterizes malignant gliomas. ABBREVIATION: TGFβ transforming growth factor beta; HMW, high molecular weight; GFC, gel filtration high performance liquid chromatography; PBS, Dulbecco's Phosphate-buffered saline; CM, glioma cell serum-free conditioned media; RT, room temperature; IP, immunoprecipitation; DMEM/F12, Dulbecco's modified Eagle's medium: Ham's Nutrient Mixture F-12 medium, βME, βmercaptoethanol; NRS, normal rabbit serum. © 1994 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.